[Twan]

If you have the full game there are spells affecting the entire battlefield and doing dammage to non protected troops.

In example you can give a staff of storms to an air mage (or cast a storm spell) and make him cast resist lightning then wrathfull skies. It's the best spell if you want to see a mage destroying an entire army (but it will affect all troops not having a resistance against lightning, including yours).

There are also the same kind of spells in other paths, fire storm (use fire fend/warriors of muspelheim to protect your troops before casting it). Foul vapors for poison, grip of winter for cold (doing stun damage).

All these spells are the best way to win assymetric wars (especially against the AIs), a mage alone (or better with a little army of troops immune to his element) can with some gems destroy hundreds of unprotected soldiers.