Poison cloud aimed at the feet will kill any monster in one turn except for the dragon king. Chain Lightning is devastating on a group and has a very fast rate of recovery. Turn undead is fantastic in the Chasm, Lich Lab and Verhoffins ruins among other places. It's the only way a level 10 party can survive the chasm. Soul Drinker is cool. Cast enrage on a group of Lizard Orcs in the training hall and watch them fight. Use paralyze on the big guys in Dook's basement (it's a good spell for the dragon king too). You can get the sparks spell up to 12 sparks of 30 points damage. Etc.

I like to get in a fight with 20 or 30 monsters and rock the room with chain lightning. It's a blast. Very effective and the coolest spell graphics. The damage report can fill your entire screen as you get multiple hits on several monsters. They drop dead in groups.

There is NO way you can clear a room with weapons in 1/10th of the time that the spellcasters can clear it. The final battle takes hours of game clock with weapons, it takes about 10 minutes with 4 chain lightning casters.

I can get in turn based mode and cast poison cloud at the feet of 4 king lichs. They all will die without getting a turn to retaliate.