[MaxWilson]

Yes, I was going to suggest Antimagic. It should cut your losses by 2/3.

Also, remember that it costs gems with all the micro-management that entails. It might be worth hitting the army in question with Arouse Hunger/Call of the Winds/etc. spells on the turn before you attack it (or it attacks you), in the hopes that casters will use up their death gems before they hit your main army. This will work better against an army on the offensive, because it's harder to replenish gem stocks.

Darkness drops precision and so would probably reduce damage from this spell, but also helps undead and so is probably counterproductive. Storm might be worth trying, though.

-Max