[Micah]

Storm should lower the caster's precision, so it might make it miss your army...that said, it's generally aiming for a fairly huge clump of troops, so a little deviation isn't going to help you much. Antimagic is the way to go if you're the defender (you'll get first cast). If you're attacking the damage will probably already be done by the time you can actually cast AM. Max's suggestion to send in decoy ritual attacks is a good one, although you can't use undead spells to do it. The undead are immune to it, and I think the AI is just competent enough to not cast it at them. Spreading out your troops and putting your junk troops up front is also helpful, but lone decoys usually won't work. Shadow Blast is one of the spells that can turn a large battle on its own, though (The other one that comes to mind is flaming arrows).

Killing the casters is also a really good plan if you can pull it off. Earthquake if they have low defense or rain of stones if they're unarmored will work pretty well against anything with human-range HP. Assasins with trample (skel spam will pretty much kill anything without) may also work. Dropping a vengeance of the dead will probably work well too after they've done some damage if you can beat their MR.