Spell Info
Spells are arranged by type.

Magic Based

FLASH:
Damage depends on spell AND char level{Formula too complex to bother;)}
Min casting cost: 16, reached at slvl 8.
Ok, yes, I know... This spell is magic-based, and so it kills Azure Drakes on norm and nightmare like flies (they don't even resist magic...). It works only on the squares surrounding you. It only deals good damage at high levels. And it's only mana-effective if you hit several enemies at once. This is a fairly good spell for co-op play. Use the tele-flash method on Balrogs and Azure Drakes, your companions in hell ill appreciate your restraint of the dreaded Chain Lightning.
Rogues: If you have high enough mana use Flash as needed.. otherwise, use your bow or other weapon.
Warriors: If you have AC 280+ and just happen to have forgotten your sword... But then again, at levels where this spell is interesting you'll probably deal more damage with your bare fists...

BONE SPIRIT:
This spell NEVER kills. That's why most consider it useless. They couldn't be more mistaken :)
Damage: 1/3 of target's hitpoints. That's 1 point out of 3, or 1000 out of 3000!!!
Min casting cost: 12, reached at slvl 13. Note: This spell costs also 6 life to cast (so 6 additional mana with Mana Shield up)
It's a ranged, magic-based attack. It goes around corners and seeks out enemies on it's own. I have to admit, magic is the most common immunity. BUT: all non-immunes get severly weakened. And the spell can be used to lure out single enemies from a safe distance. Try it on Azure Drakes on normal/nightmare ;)
Rogues/Warriors: Useful as well in luring out single enemies...

BLOOD STAR:
I admit, I rarely use this myself. But it's not COMPLETELY useless ;)
Damage depends on spell level AND magic skill {3*slvl+MAG/2-MAG/8}
Min casting cost: 14, reached at lvl 7. (Also costs 5 life to cast - 5 mana with shields up)
It's ranged, single target magic-based. Damage is not too hot either Most critters resist magic or are immune (apart from the often mentioned Azure Drakes ;)...
Rogues: Damage depends on magic skill... Arrows do more damage and there's no resistance ;)
Warriors: See above. Use a sword :)

Lightning Based

CHARGED BOLT:
Your first lightning-based spell. Cherish the staff you carry until you learn this spell for yourself.
Damage depends on your magic skill {1 to (1+{[MAG/4])}, number of bolts per cast on the spell's level {3+slvl}. Minimum casting cost: 6, reached at slvl 1.
This spell is very efficient at the beginning. Again, many forget it when advancing, because in most cases, Lightning and CL seem more effective. True to a point. But: at high levels, you'd be surprised how much damage this spell really deals, for lousy 6 mana. Roughly 1-40 damage, times 20+ bolts, I call that a deal! Even in hell/hell this spell can be used to flood rooms with it and severely weaken lightning-susceptible enemies. Not to mention, filtering them out of a mixed pack. Rooms full of Balrog-type baddies can actually be KILLED with it, as their erratic movement does NOT protect very well from charged bolts criscrossing between them.
Rogues: Useful in the beginnning, as a "front-runner", to weaken enemies for your arrows and flood small rooms. Low magic skill makes it less useful later.
Warriors: See rogues and add even slower casting speed and lower magic skill...

LIGHTNING:
The mages prime lightning based attack before Chain Lightning goes up in lvl, and in coop fights.
Damage depends on character level alone. {2 to (2+clvl)}, but the length of the bolts increases with spell level. {4 +slvl/3} (Caveat: it seems that the single sprites actually deal the damage, thus a longer bolt would do MORE, and the effective damage done would always be a multiple of the number given in the spellbook.)
Min casting cost: 6, reached at slvl 5
What can I say? NEVER forget about this spell. Even once you have Chain Lightning. It pays to have mastered plain Lightning well. Your teammates will thank you. Learn to cast it along walls, after you rounded the corners. Line up enemies in a row to maximum effect. Even in hell/hell this spell, combined with stone curse and good timing, drops rows of Balrogs and Hellspawn quick enough, without frying your buddies.
Rogues: Hmm, rogues might reconsider, and use Chain lightning for mobs and arrows in coop situations.
Warriors: ...kill faster with a good sword at higher difficulties...

CHAIN LIGHTNING:
The most mixed blessing in your books. One of the most powerful AND one of the most dangerous spells. Learn to control THIS and you've mastered the art.
Damage depends on Character level ALONE {4 to (4+2*clvl)}. RANGE depends on spell level {2+slvl}. Number of bolts fired: 1 plus 1 more at EVERY monster in range. Length of bolts also depends on spell level.
Min casting cost: 18, reached at lvl 13.
Lightning bolts sizzle out toward your enemies (the number of bolts appears to depend on your level, the spell's level and the number of enemies) and electrocute them. When using the spell, try to make sure that all of your enemies are in the same direction away from you. Otherwise, the bolts will shoot off in many directions so that each creature may be hit by only one bolt. If the monsters are packed in together (like in a narrow hallway or a doorway), each one may be hit by a dozen bolts or so. Unless they are lightning resistant, they will go down quickly in these cases.... One thing to note is that the spell will track creatures who are not visible; that is, they will fire toward a wall that has monsters behind it. This can be useful and annoying. You can use it to assess whether there are a large number of monsters beyond that wall or doorway. However, this factor will pull bolts away from enemies that could be hit. You can counter this problem and use it to your advantage. When facing your enemies, make sure they are between you and the wall I just described. The enemies behind the wall will cause more bolts to be fired into the mass of your assailants!  This spell is not so good for multiplayer. Even playing very carefully, you will often fry your friends. I almost always remove it from my hotkeys when playing with other people. If you split up, though, the spell is back in the game. If Lava Maws are in a game, I find the benefit of using the spell to be greater than the cost of not having allies close by me.... A final caveat: Chain Lightning uses up a lot of sprites. At higher levels, you will see many "breaks" in the lightning that issues from you, so that you'll miss enemies who are somewhat close to you. Many people prefer to keep the level of their chain lightning spell below 12 or so. (Enchanted Shrines almost always lower this spell, so try finding one of them to lower it if you need to do so.)
Raw stopping power. Self-targeting. Awesome damage at reasonable cost. What could possibly be wrong with this spell, you'd ask? Three things: At high levels, it targets monsters you can't even see. So if you are not close to a dungeon corner, it's very likely that bolts go to unexpected directions (and your buddies might stand there...;). Note: On the other hand this can act as a kind of "sonar system".. It shows the presence of large numbers of enemies behind walls... It also targets every monster in a large room. If you stand in the center, every monster gets only one bolt. That won't kill, but will make 'em VERY angry... And as they come close you might discover the major drawback: There can be only as much magic in the world. If too many bolts start at once, they get gaps. Loopholes. Points where there IS no lightning. Monsters will slip through, and possibly swarm and kill you. (The sprite limit makes some mages want CL at lvl 11, I prefer to max CL and control it. ;) But if you learn to channel your foes, make them follow you around corners, through narrow passages, or if you master the art of arranging them with Stone Curses, this spell will be devastating. If the main force of enemies is in the same direction, multiple bolts will stream as one towards them, killing everything with one cast. Learn to control CL. See the mage strat page for controlling CL.
Rogues: A good CL can help a lot to weaken groups of enemies...
Warrior: I don't bother at hell difficulty... Swords kill and there's no resistance against them. But can be useful against some enemies in norm/night...

Fire Based

FIREBOLT:
Your basic method of attack at lower levels.
Damage depends on spell level and your magic skill. {Formula: (1+slvl+[MAG/8]) to (10+slvl+[MAG/8])} Minimum casting cost: 3, reached at slvl 7.
Also, the speed of the bolts increases with spell level. So at high levels, damage will usually be around 60-70. Seems ridiculous, compared to the 1000+ of a high-level Fireball. And a Fireball also has Splash Damage... Most folks stop using this spell once they know Fireball. (Though one might reconsider: a high level Firebolt is faster, deals good damage and might actually in many situations be better than a LOW level Fireball...) Why should one bother? Well, IMO, because Fireball also has splash damage. If you wanna join the battle in a coop fight, many Warriors get _a bit_ upset when you shoot Fireballs at a critter standing next to them... So if you want to help with the kill {trans: gain some exp ;)}, a good old, lowly Firebolt (or 10... or 20...) can come in handy.
Rogues: forget this spell. You hit better (and faster) with an arrow, deal more damage (your magic skill is much too low) and no critter resists them :)
Warriors: forget it... almost. You cast too slow and hit too bad to be effectice with it. Damage with you magic skill stinks. BUT: It _can_ be a last resort to bring down some weak enemies that have ranged attacks. But doesn't work in higher difficulties any more.

INFERNO:
This is probably the most useless spell of' em all.
Damage depends on clvl { 3 to (6+ 3/2*clvl)}, contrary to belief, area of effect seems NOT to increase with slvl.
Min casting cost: 6, reached at slvl 6
Hmm... it's fire based, it's slow, it requires the enemies to be close. Do I have to say more? It can hit several enemies at once, but only if you give them time to accumulate. Right in front of ya... A fireball also hurts several critters, and does so at range. Damage isn't great either. IMHO, this spell is a no-no...
Rogue: Can you say "Bow"? :)
Warrior: Can you say "Sword"?...

FIRE WALL:
The #1 support spell at low levels, and the most net damage for you mana.
Damage depends on character level {(4+2*clvl) to (40+2*clvl)};
Duration depends on spell level {8+8*slvl seconds}
Min casting cost: 16, reached at slvl 7.
Early on, Fire Wall is simply THE best spell that kills the most enemies. In church and cats it can slaughter rooms full of monsters. Learning to position it asap is a must. The wall will allways "try" to appear at a right angle to the line between yourself and the point where you aim. It's 11 squares wide, unless it hits a wall. So learn to position it carefully. Stand in a straight line to the point where you want it, and aim along the dungeon's axis. If the line between you and the target point is not along the axis, the wall might appear diagonally and hit you, your comrades, or miss the monsters. Note that it is possible to walk through a wall diagonally without taking damage more often than not. This applies to critters, too. Casting one behind you can cover your retreat, casting one right in front of you will make attackers (Like Mr. Diablo) stand in it while hacking at you. Several Fire Walls can be "stacked" on top of each other for ADDED damage. But watch out: if you cast too many of 'em, you might find you cannot cast another spell anymore. The world can only handle as much magic at a time. (The sprite limit Diablo has is easily used up by multiple Fire Walls. Don't overdo it, or your Fireballs will not come anymore ;) At higher difficulties, as the monsters become tougher and more resistant, damage will not quite keep up, and the usefulness will somewhat decline. But it can still be used to fry some witches :)
Rogues: The main reason this spell is incredibly useful: it burns while you do something else (like shoot arrows). As aiming and casting time are no major concern, this spell is one of the best for rogues.
Warriors: See rogues (and replace "shoot arrows" with "swing a sword"...

ELEMENTAL:
The masses eschew this spell, for no reason I can imagine. Think of it as a Fireball that can GO AROUND CORNERS and hits on it's own...
Damage depends on spell AND character level {Given damage is the same as Fireball at equal levels, but it actually does less, and seems to do less splash damage}
Min casting cost: 20, reached at slvl 9.
Ok, it deals less damage than a Fireball and costs twice as much. BUT: I don't have to aim and I can fire it from a safe position. Have you ever killed a huge pack of Advocates, by standing around the corner, whistling a merry tune and sending Elemental after Elemental in?? Without getting hit ONCE, while, had you gone in and Fireballed away, you'd have become TOASTED? Have you ever drawn out one enemy after the other from a huge room, without even showing your face at the door? Have you ever, while on the run, cast spells IN FRONT of you, that turned around and fried the pursuing Blood Knights? No? Learn to use Elemental ;) Sometimes it even runs right through a weak enemy, killing him, and continues to hit another one.
Rogue: A spell that doesn't have to be aimed comes in handy.. Especially in combo with a bow.
Warrior: Still a nice spell to draw out the Knights in hell/hell...

FIREBALL:
Yep that's it. Everyone's favourite... Raw dumb firepower. The most damaging spell in the game...
Damage depends on Char AND spell level {formula is kinda... complicated ;)} Note that it also does "Splash Damage", i.e. all squares adjacent to the target seem to receive 25-50% of the damage as well. It is yet uncertain, but it seems the target itself gets this additional damage as well.
Min casting cost: 10, reached at slvl 7.
Right. Yep. Heavy Artillery... At high level it kills everything that is not IMMUNE to fire in a few shots. Kills everything that is not RESISTANT to fire in one... Plain and simple ;) It still pays to master it's use, though. Learn to shoot at straight lines. Learn to aim. Learn to cast it while dodging the enemies spells... (Ever killed a room full of witches without getting hit, just by running up and down in front of them and throwing Fireballs? Learn "Happy Feet" ;) And combine it with other spells to best effect. Stone curse to aim better. Guardian (or Elemental) to draw them out to a good "shooting gallery".
Rogues: Depending on situation, see what kills faster: A Fireball or some Arrows. But you'll need some enchanted shrines to raise the spell level.
Warrior: Even more shrine hunting necessary to make it useful. You'll have a lot trouble hitting something at higher difficulties. Use the Stone curse targeting marker mentioned above if nothing works...

FLAME WAVE:
Basically, a fire wall that moves away from you and toasts everything on it's path. Or not everything it seems...
Damage depends on char level {(6+6*clvl) to (60+6*clvl)}, number of flames on spell level {5+slvl/2}
Min casting cost: 20, reached at slvl 6.
This spell is nice, but it also has some major drawbacks. First, it's expensive to cast, the damage seems to be split among the flames, thus it needs LOTS to kill something in higher difficulties. And: moving targets tend to get hit less often. Like a firewall one can walk through w/o taking damage. Apart from that, it can be nice to sterilize a corridor, to draw out non-fire-immunes (watch out! sometimes it draws out more than you bargained for! ;) or to have it as a front-runner... If you hear some ouches, switch to Fireball ;)
Rogues: The cost makes it a bit less interesting here...
Warriors: the casting cost kills it, IMHO. Not worth using for a Warrior.

GUARDIAN:
One of my favorite spells. Incredibly useful as a sniffer, a filter, a shooter... Even as an avenging angel should you die...
Damage depends on Char AND spell level {too complicated to bother ;)},duration on both, too {max: 24 sec}. (Note: speed of the bolts may depend on your FIREBOLT spell level. Damage might, as well, still uncertain...)
Min casting cost: 30 reached at slvl 11.
Many disdain this spell. Many underrate it. A low level guardian doesn't deal too much damage and is expensive to cast in the beginning. So a lot of people haven't noticed: A high level Guardian is at 30 mana a steal and a half!! An average "normal" sorcerer at really high levels has no problem to cast 30 Guardians from one mana ball. Can you imagine what 30 lvl 18 guardians DO to a room full of non-fire-immunes??? ;) A Guardian sees invisible foes. It's the BEST defense against those pesky Illusion weavers. Cast one ahead of you as you walk along, throw in a CL as soon as it fires at empty air - voila... A Guardian rarely misses. It's a fine spell for coop situations. Just don't let your buddies get between the Guardian and the enemies... (Important: YOU are immune to your own Guardian!!!). A Guardian keeps shooting while YOU do something else (that even includes lying dead on the floor :). EXCELLENT support fire. It even STUNS many enemies! (Makes 'em easier to Fireball ;) A Guardian can recon around a corner. Cast it behind the doorway and wait what it does. If it fires and you hear screams... get ready to fireball what comes around the corner. If it fires and you hear nothing... break out that CL... That way you can filter a pack of beasties with mixed immunities without showing your face... A Guardian improves your effective range. Cast it (or a lot of them...) at the edge of your vision, and if fires SEVERAL squares farther. Problems with spitter bosses? LOL! :) And if you count the number of shots a high level Guardian fires in 24 secs, the damage it deals is not bad at all :) 8 of them kill hell/hell Blood Knights with only a few salvos... Remember, in a co-op situation, cast Guardian BEHIND the monsters, so the firebolts will not hit your companions.
Rogues: Everything remains true, especially the Guardian/Arrow combined fire can be AWESOME at high levels.
Warrior: The high mana cost makes it a bit less useful. Mainly for drawing out enemies from a group (Knights on 16, for instance) it's still nice, though.

Other Types

HOLY BOLT:
This affects only undead and Diablo himself. BUT: it is neither magic, nor lightning, nor fire. Resistance is futile!
Damage depends on clvl alone {(9+clvl) to (18+clvl)}, but spell level affects the speed of the bolts. Min casting cost: 3, reached at slvl 5.
As a mage: don't tackle King Leo without it :) The spell is incredibly useful against all types of undead (no resists!) and Mr. D. Note that it is impossible to stunlock Diablo in hell difficulty with that spell (You'd have to be lvl 54 to be sure :). In norm it's a sure kill if you're lvl 24, in nite you'd need to be 39 to stun him with every hit.
Rogues: Test for yourself what kills those pesky skeletons faster: your arrows or HB (depends on bow quality etc.). Leo is A LOT easier with HB, even though you're not as fast with it.
Warrior: For skeleton archers and Black Deaths, this spell can come in VERY handy, even at your slow casting speed.

HEALING:
Well, ok, I consider this spell a necessity until one finds a Book of Mana Shield. After that - forget about it ;)
Efficiency depends on your level, the spell level and a a hidden bonus {([clvl+slvl]*bonus] to ([4*clvl+6*slvl]*bonus)} (The bonus is 2 for Warriors, 1.5 for rogues and 1 for sorcerers)
Mana cost INCREASES with your level and DECREASES with the spell level. {8+2*clvl-3*slvl}
In emergencies I prefer to heal with potions from my belt, and continue to cast offensive spells while doing so ;)
Rogues: Depending on your style of play and equipment, decide for yourself if you go with Mana shield, or healing with spell, or gulping big yellow pots...
Warriors: Get this spell up. Get it up some more. And then some.

HEAL OTHER:
Your basic spell for being nice.
Efficiency and casting cost go along the table for healing, with a 10 point "being nice" bonus, it seems ;)
Not that useful of a spell, but it is friendly. I rarely cast it on anyone during combat. It can be hard to target another player with it when he is in the middle of a mob, and it can also distract them. However, I use it in two cases: I accidentally hit them with an arrow or spell, or we finish a battle and he/she has some hit points missing. Note: avoid casting this repeatedly on anyone who is surrounded. If they are stun locked, it may be better to let them die than to keep them alive. The reason is that their equipment will be taking damage along with them, and they may lose a great item instead of coming back to get it off the floor after dying....
Rogues: If you know nothing better to do with your mana, be nice to the Warrior...
Warrior: Your bonus can make casting this worthwhile, staves of heal other, if actually in use early in the game, are best given to the warrior thus(ironman tourneys).

TELEKINESIS:
The spell has three uses: open doors/chests/etc from afar, pushing away enemies and picking up items from a distance... only one of 'em is really useful.
Min casting cost: 8, reached at slvl 5. (No need to ever go higher...)
This spell is very handy for extricating your lost equipment from a mob of monsters. In case you haven't noticed, the spell will work through walls. This means that you can hide behind a wall, cast telekinesis and grab things off the ground. When Balrogs are walking all over the place you died, you will appreciate this spell greatly! Another small benefit is that you can use the spell to activate shrines, open book cases, open chests, etc. If you don't feel like clearing a room (or you're not powerful enough), just use Telekinesis to grab the stuff there. Use it to open trapped chests so you don't get hit by Nova. One little trick I tend to do is to use the spell to activate Murky Pools. When you touch a Murky Pool, you get temporary infravision (don't you just wish this would last an entire level???). So, if you use the spell to "touch" it, you can now see what monsters are in the room with the Murky Pool.
Rogues: Same as sorcerer, perhaps more so
Warrior: As they have the most difficult time retrieving items naked, TK is actually MOST important for them...

TOWN PORTAL:
Not much to say about this spell... Everyone who's not a marathon man should learn it asap ;)
Min casting cost: 18, reached at slvl 7. No need to go higher.
Well, it shortens trips to town and saves time... The only thing to mention apart from that is: It creates an alternate entrance to the level you're on, should you die close to the stairs and not be able to retrieve your stuff. Always cast a TP in the first safe corner of a new level, away from the stairs. Try never to flee towards it.
Nothing to change for rogues or Warriors ;)

STONE CURSE:
The single most powerful spell in the game. Period.
Duration is based on spell level {4+slvl sec; maximum: 12}
Min casting cost: 40, reached at slvl 8 (and thus, no need to have it higher)
This spell works on all monsters but Diablo himself. It effectively neutralizes them for 12 seconds. It stops them from hitting you, from advancing, from freeing a blocked passageway and makes them almost impossible to miss with weapons or spells. Do I need to say more? Master this spell at all costs. It's your best armor. Your turbo switch for killing moving monsters quickly. Your best friend in teamwork. Learn to switch between SC and offensive spell in the blink of an eye. With SC-lightning-SC-lightning you can drop even those dancing Balrogs in no time. Killing an Azure Drake with one SC and ONE Fireball is faster, safer and thus more mana-efficient (you take less hits on your Mana shield!) than killing it by casting 3-5 Fireballs and missing a lot. Not to mention the misses might end up hitting someone else... ;) Also a great co-operative spell. If your friend is surrounded, turn her enemies into a statue garden! And stone curse is your best friend against triple immune monsters. Stone Curse and Golem, or Stone curse and (bleaagch!!) a weapon will kill them. A master of this spell will aim and time it so that the enemies end up arranged in rows (for lightning) or packs (for fireballs)...
Rogues: Stone curse and arrows can be VERY effective ;). AND it's the best spell against those tough boss monsters.
Warriors: Next to teleport the best bet against witches, though MUCH more expensive. But it's aiming-aid-function can be nice: A Warriors Fireball does as much damage at the same spell level as a sorc's. BUT: in hell difficulty you miss alot (due to low magic skill)... So: If you face a pack of snow witches try this: Stone the center one, throw a fireball at this "marked target"... watch them die ;)

PHASING:
The poor man's Teleport. And a poor Teleport, IMHO. I almost neverused it.
Min casting cost: 4, reached at slvl 5.
I never bother with this random invitation to a screaming death. It takes you away from point A and teleports you to a kinda random point B (in one of 4 directions). Point B can be safety, point B can be in the middle of 6 Lava maws... Decide for yourself. Some consider this spell an emergency Bail-Out possibility (casting it is faster than Teleport, thus more likely to work in stunlock)... I say, walking a few steps and casting Teleport to a point _I_ choose, or avoiding the emergency in the first place is much preferable. One use for this one might however be: If you died close to the stairs or town portal, and have NO way to recover your gear, this _might_ be the spell you cast fast enough to get a bit away from the enemies and raise your Mana Shield or cast something else... Just cast phasing the very moment you enter the level, and: good luck ;)
Rogue: Oops, right into that cauldron!!
Warrior: Pha..WHAT??? Are you kidding? ;)

MANA SHIELD:
In Hell you'll die without it. Any more questions? Ok... ;)
Min casting cost: 33 at lvl 1 and ever beyond.
Since the 1.07 patch, any level of mana shield decreases the damage you take from an attack by 33%. (Prior to that, lower levels of mana shield decreased damage taken more than higher levels did.) And what can I say? It makes you use your mana pool as your life, and thus allows you to survive more hits than even the sturdiest Warrior. Raising the shield should become a reflex upon entering a new level. Never let it drop! If it drops, you're dead! For emergencies, carry a Big Yellow pot in Belt#7&8 {Hotkeying MS is a waste, IMO. Learning NOT to let it drop and using the "Oh my god I'm gonna die!" - keys for emergencies is MUCH better :)}
Rogues: It's a matter of your personal style and preferences if you go with or without MS. Depends on equipment, too. With lotsa mana and low life you might want to go WITH MS. Otherwise, mana might be too valuable, and healing more efficient...
Warrior: Don't bother. Your pitiful amount of mana is way too valuable.

GOLEM:
A sorcerer's best friend and "weapon" of choice against triple immunes. Not MUCH more stupid than some Warriors I've met.. ;)
Stats:
Hitpoints: 2/3 maxmana + 10*slvl
AC: 25
To-Hit: 40 + 2*clvl + 5*slvl
Damage: (8+2*slvl) to (16+2*slvl)
Regeneration: 1.9/4.1/6.6 hp/sec on norm/nite/hell
Min casting cost: 60, reached at slvl 8.
Well it seems simple enough. The earth comes to life, walks toward any monster it sees (even opens doors on the way) and bashes away... And if there is no monster in sight, it walks in the direction you look. Two golems by two casters seek each other and fight to the death. Casting it a second time will destroy the golem. Hmm... Simple? The finer points are tricky enough... A Golem has an kind of "infinite light radius". All monsters within line of sight "feel" it's presence. They don't wake up fully, but ranged attackers WILL fire at him! This can make walking alongside your Golem EXTREMELY dangerous... If a Golem attacks a monster, it will become fully activated. The Golem doesn't hold well against high-level melee monsters. It's extremely fragile against witches' Blood Stars. Fireballs kill it quickly (your own, too ;). Lightning, OTOH, seem not too devastating. And The flash spell from mages seem not to bother Mr. Golem that much... The Golem is most effective against stone cursed monsters, smashing the statues quickly. Advocates are killed easily as well, as they do not shoot Fireballs at him, only flash when the Golem hits them. For killing Obs Lords: SC and Golem, for killing Advocates: with some patience the Golem can do it alone (stone cursing speeds it, of course ;). Killing Soul Burners on hell/hell is a science in itself. Don't let the Golem advance. He dies in a blink from their Bloodstars. You might catch the ricochet... Advance slowly yourself. Activate A FEW witches. Dodge their attacks and retreat to a safe corner. Stone them all. Cast Golem next to them. Keep them stoned while he finishes the masonry work. Destroy the Golem as soon as he's finished. Rinse and repeat ;) Another great use for Golem is to clear around Laz's chamber, pull the witches out and kill them, then cast him in the chamber, he will go to work in the chamber, while you clear the rest of the level (perfect for Hell difficulty)
Rogues: It can be fun to let the Golem run amuck in lower levels (letting him clear church on his own ;), but apart from that: Don't bother too much. His hp are low (due to your max mana) and you have a bow that kills triple immunes faster ;)
Warrior: The Golem walks towards enemies and whacks at them. Remind you of someone? ;) Don't bother with the 60 mana...

TELEPORT:
An extremely useful spell in several ways.
Min casting cost: 15, reached at lvl 8. (No need to go higher).
It allows very quick escapes when you get into something you can't handle (as long as being hit doesn't prevent you from casting it). Teleport through a wall (back into explored territory, of course!) to get away from those rampaging knights and flying bloodstars. Once you are a powerful, competent Mage, you can teleport around a level, killing as you go. The spell also allows a Mage to keep his distance from incoming attackers. Cast a few spells, then teleport half a screen away from the attackers, then repeat until they are all dead. If you don't have enough explored territory with which to do this, "leap frog" over your opponents: cast until they get close, then teleport over them. Cast some more, then jump over them again. You'll keep them confused and keep yourself alive. Note also that a teleporting target is very hard for ranged attackers to hit.
Rogues: Teleport is a even MORE useful for a rogue. A rogue with bow kills slower, has to retreat more and uses fewer spells. Very useful with the "leapfrog" strategy.  She should also have a free hotkey for teleport.
Warrior: Get the spell UP and learn "Telekill: the great equalizer"  A MUST for Warriors, and one of the three MOST important spells for them :)

Scroll and Staff only Spells

INFRAVISION:
Can't get this one in your spell book, but the scrolls are worth the cost (600 GP) on higher difficulty settings. If I have one, I cast it as soon as I enter a level so that I can see through walls and know what enemies I'm facing before I actually face them. If you do this, be sure to report what you see to your comrades. Also, the spell allows you to Stone Curse an enemy who is behind a wall, which is helpful in many cases (e.g., Stone Curse Lazarus' cronies this way before entering his room.

IDENTIFY:
Buying a scroll of identify (100 GP) is no more expensive than having Cain ID an item. The advantage, of course, is that you can ID your item while you're in the dungeon. If it's a good item, you might want to equip it. If it's a bad item (cursed or just worthless), you can just dump it on the floor, making room for new treasure. (Only problem is... you can't take screenshot to verify the legitness of item)

RESSURRECT:
Need I say, "Duh?" If you have an ally, it would be nice to bring him back from the dead, should the need arise. This is even more true if you are the cause of his demise....

NOVA:
A fairly good spell if you come across it. Does quite a bit of damage to creatures close to you, and very little to those further away. Use them if you find them, but they're probably not worth buying. I have heard that a staff of Nova can be very fun. With multiple charges, you can clear a room pretty easily.

APOCALYPSE:
This is a powerful spell, so smoke 'em if you got 'em! It causes jets of flame to explode around all nearby enemies. It strikes many opponents and harms even those who are immune to all! That's right, blow up Soul Burners and Advocates on Hell/Hell with this spell. It requires 149 magic to use it, which can be a strain for Rogues and Warriors. I like to save these until I have 3 or 4, then cast them all at once against triple immunes. Takes a few games of hoarding them to do this.