You are forgetting one of my favourite spells : Pestilence !!!
It's like Acid Gas, only it does a little less damage, but the duration is amazing ! And it stacks ! Cast once, and you do 2 to 3 actual damage per second, for a duration of over 20 seconds, cast twice, and you're doing 5 to 7 damage per second, cast three times and you're doing 10 to 15 damage, fourth cast 20 to 30 damage per second, ... And all the wile you keep getting the insane amounts of xp you get from the fragment damage, and it deals damage to nearby targets.
This spell can be obtained from low midlevels, and stays very useful up to the point where you are casting pillars continuously with ease. You can easily use this spell in MP to rake in the levels, because you only have to cast once and then stay clear from getting hit. If you have time over, cast again to speed things up. (Very nice practice on dodging those swamp-troll clubs too.)
At the low levels, having Leech Life in slot "3" and Rip Mana in slot "4" can make you fairly invincible, too. you can keep on draining your foe's health while replenishing yours with Leech Life, and when you get low on mana, you cats Rip Mana once and switch back to Leech Life.
I absolutely loved Explosive Powder in regular DS, and in LoH it can be nice to have, too, at low level.
Once at high level, nothing gets rid of those Cave Trolls as fast as Cluster Bomb.
Corpse Transmutation is a very nice spell to lug around. At low levels, you'll be using it often, most likely right after a battle to get ready for the next. At high levels, chances are you have regenerated your mana before you have switched spells. (At least, that is for me. I have 4 full spellbooks on my mage, not including the one for crafting skills.)
Swiftness of the Dead needs no introduction. It speaks for itself.
Pillar/Cyclone/Tornado of Fire is very nice, for immobile targets. It makes short (and easy) work of people inside rooms with closed doors. You can force-cast it on the ground, even if that area is invisible on the map. Just right-click (or mouse-over and hit "c" on the keyboard) the area, and you'll cast a pillar. After a few pillars, not much remains alive in that unexplored room. That makes cleaning out the castle a lot easier, too.
(Not that the castle is really hard, but sometimes it's fun to fry some Azunites, knowing they don't even have a chance to know what is hitting them.)
Opening the doors, casting Pestilence once on each Bishop, leaving the room again and closing the doors can be fun too. Just wait 10 to 20 seconds and you'll hear them screaming their last. I usually think of it as a game of tag... You tag the Bishop with Pestilence, then hide (close the door between them and you) so they can't tag you back. Sometimes you have to back off away from the doors, because they have been casting pillars.
Flash is indeed one of the best low-level NM spells, though in LoH you might want to get your hands on "Summon Deer", too.
(That spell can be ever so much fun to distract new players... ;D )
And I actually do like buffing, depending on the occasion.
Spirit Armor is a very nice one to have, at high level. I think at level 150 CM, it will add 100 armor. And 100 armor is fairly high for mages. Especially since Spirit Armor's cost is low and duration high.
Flame Shield is very handy to have, especially in areas with rats and bats. Those little pests otherwise may keep you occupied instead of facing the real dangers. Flame Shield takes care of them.
Death Aura is also very nice, if you don't mind to run around with a "continuous-taunt". The person with Death Aura will always be the primary target for the monsters (due to the fragment-damage it does), so it may not be a good thing for a mage in the company of melee characters to use.
And to tackle Fury, Invincibility is helpful. It add something like 1500 to defense. The duration is really short and the mana cost is high, but with enough mana-regeneration you can get it to something manageable.
Summon Minion is very useful against Fury too. Especially if you can give him all "health per hit" equipment you can get your hands on, and the highest armor he can wear. Then you can send the minion first, to tackle Fury, while you can stay behind to support your minion with Battle Healing and Cluster Bombs, from Fury's back, a bit away so you don't get hurt as much from his area-of-effect-damage attacks. And, of course, casting Curse and Malign takes some of the bite out of him. Don't waste you time and mana on casting Weaken; Fury has no melee attacks, so Weaken doesn't work on his attacks.
Mana Shield is very helpful for the characters who don't use mana during battles. Of course, chances are these characters don't have high enough level NM to even cast this spell, or have enough mana to make it worth it. (I only use Mana Shield on my Valk, when I'm sure I don't need the mana for healing. Of course, if I had a healer nearby, I'd probably be using Mana Shield a lot more often.)
Nature's Transformation.
You can't believe how often this spell has saved lives in my MP games. Occasionally I have one of my friends/family who want to play a game of LoH with me, even though they are fairly unfamiliar with it. So on their first few hours of LoH, they often get into quite unfavourable situations, and almost always succeed to get me directly involved too. Usually I try to flee before the monsters all mob me (after the others all died), so that I can sneak back with Cloak of Shadows and revive the others, but very often I don't get the chance to run. Nature's Transformation may not always work, but more often than not it does work. It gives me the time to regenerate enough health to survive long enough for casting the revive spell/scroll and bolt.
Everywhere you go, for a caster a Magic Chest/Gnome can be helpful. Especially if you need your minion to wear good equipment. Minions don't keep their equipment, so after use you want to store it in the Box/Gnome for when you have need of the minion again.
Summoning a Valk is also nice, to compliment your minion. That way you have two strong summons to beat on Fury. If you keep healing them, at some point you're bound to win.
Summon Army of Darkness can be fun, too. If your mana can keep up with it. I'll try to get a nice picture of such event at the screenshot thread.
