[Wish]

a bunch of priests set to banish take out priests pretty effectively.

additionally, learn some thaumaturgy and get some of those astral mages set to cast mind blast - it won't target mindless troops (like most of the cheap undead)

Pythium has the advantage of really getting a lot of gem income, and powerful mages. they don't really get archers, so you have to research some evocation and use those spells. Unfortunmately in the demo, you can't really reach the good evocation spells.