Since this thread now seems to be more about which spells are useful
: I feel one spell should be mentioned as well: Fear.
If you only use level 3 and up units, it's really nice to cast fear on those huge stacks of low level creatures. I don't think I would have survived that stack of 2000+ fireflies (when fighting that dragon at the Orc's castle) at level 11 without that spell. Well, not without losses anyway
I've only just started using Inquisitors with Sacrifice, and even while playing on Impossible (mage, now lvl 12), it feels a bit like cheating
. Sacrifice that giant, and resurrect him for free units.. It's a bit too easy.
So far, important components were:
low level mage: mana spring & fireball, pain mirror in heavy fights
low-mid level mage: fear & slow, sacrifice if I must, damage spells if I have the time