After reading through the complaints that various players have with how difficult some enemies in this game are, I'm beginning to notice something: pretty much every problem mentioned can be solved by magic.
Playing through the game as a wizard myself, I've noticed how easy it is to take out enemies who are a significantly higher level than me with the right spells, including even the final boss. So, for the benefit of all players everywhere, I'm listing the six basic magicks that will allow you to overwhelm everybody. I recommend putting maximum ranks in all of them.
1. Meteor Strike: A basic fireball spell. Not always accurate, but easy to spam and effective against all non-metallic undead.
2. Lightning: Exactly what it says on the tin. It's more expensive than Meteor strike, but it does have one game-breakingly powerful advantage; you can cast lightning bolts about as fast as you can click with your mouse, and enemies struck by a bolt almost always flinch. Judicious use of the pause button, plenty of mana potions, and a quick trigger-finger can enable you to destroy non-resistant enemies at a distance while they're too busy writhing in pain to fight back, Emperor Palpatine style.
3. Freeze: Enemies stop coming at you. Liberal application of the freeze spell and pause button can ensure that no one can touch you. The damage isn't much, but a motionless target is an easy target for other spells.
4. Slow: Freeze only works on enemies who don't have significant spiritual resistance, but slow works on everyone. A slow spell on a normally super-fast enemy will halve that foe's movement rate, attack rate, and spellcasting rate. When freezing doesn't work, slowing the foe will usually suffice.
5. Spike Growth: A spell that is easily overlooked because it does physical damage, and that's what Swords are for, right? Don't be so quick to judge. Spike growth is extremely valuable towards the end of the game, when you start encountering large numbers of enemies who are resistant to other elements. Plus, as you level it up, the width and breadth of your spike-field grows... and your spike field cannot hurt you, making it handy for eliminating both enemies at long range and at short range. Unlike a sword, an enemy standing in your spike field cannot be missed.
6. Skeletal Wall: Summons five skeletons. This spell is a sight better than those wimpy summoning spells that only let you call up a single badger, or whatever, primarily because your group of skeletons is better for attracting the enemy's attention. They might not actually KILL anything, but large groups of enemies will split their forces to attack the skeletons instead of you, leaving you free to use your other spells to destroy everyone.
Lead with Skeletal Wall. Next, use any of the following simple yet devastating combos:
Freeze/Slow + Meteor Strike
Freeze/Slow + Lightning
Freeze/Slow + Spike Growth
There are NO enemies in the game that cannot be handily defeated by one of the three combos listed above. Just make sure you've got lots of mana/restoration potions.

Great observations . . .
Just as a reminder for folks just starting the game, opening your skills, hovering the pointer over a skill and pressing an F Key allows quick access to skills without the need to pause. Once you are familiar with your F key assignments, you can bring much pain to your adversaries in just a few key presses / mouse clicks . . .=)
I overuse the lightning skill myself . . .along with poison weapon . . .and freeze . . .=)

Good to know. I played through as a warrior the first time, but maybe I'll give this class a shot next time through.

Eeeh.... Now I'll just have to use the lightning skill with a new char :-D.
(Great write-up, BTW, very informative for those new to the astounding power of Spellcasting. I myself made royal use of both Meteor Strike and Freeze (the former especially in the earlier stages of the game).)