Basic beginner questions!?

[BB Shockwave]

Not only for damage spells, but also Fear rules - necessary for keeping that last level 1 unit in fear while you heal/resurrect your army.

Also don't forget about Kamikaze. Nothing better then to summon a Phantom, send it to the midst of the enemy and cast Kamikaze on it. That spell does more damage then even Fireball maxed.

Evil book can be a lifesaver early on in battles, especially in supression battles as the Gremlin towers will inflict almost zero damage to it. The description of the book is buggy, though - you don't need to have it eat one of your units, simply attack an enemy with it and it can cast again next round.

Plague is nice too if you are using an undead army. And curse on level III.

Oh and the best for the last: Sheep. You can kill even tough units in those 2 rounds while they are sheeped...