The spells I use most are:
Priest - Insect plague, completely disrupts enemy mages
Mage - Magic Missile, although Chromatic Orb is better once you reach higher levels
Defensive spells are also a must, I always use bless and chant if I know a battle is coming up, stoneskin and haste is good to include in your preparation casting routine.
I agree with Melusine though, that it's the overall strategy which is most important, and using good combinations of spells.
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Time is an illusion. Lunchtime doubly so.