[VedalkenBear]

I was actually suggesting both. If you have Astral mages who aren't priests, they're more damage. Of course, if you have Thaumaturgy spells, then you might be better off trying to use those, or making use of them in other ways.

To take the magic types in order:

Fire:

Battle Evocations. Falling Fires is probably the best I can think of offhand pre-7. With a high-level mage, this will work very well against Longdeads. There are several other options. With Marignon/Flambeaus, Holy Pyre is an option.

Air:

Air is better at smaller, quality-based armies. Still, Wrathful Skies is probably just as good as other options. Thunderstrike is also pretty good for area effects.

Water:

This is probably the weakest magic type vs. Undead. Ice Strike is okay, I guess, but it's Evo7. If you have Water/Fire mages, Acid Rain is a good choice. At Evo-6, you get Cleansing Water.

Earth:

Blade Wind against the real chaff (i.e., the ones without shields). As others have said, Rain of Stones can be effective. Magma Eruption is also a good choice. A somewhat cute trick would be to use something like Destruction followed by Blade Wind. Probably not too practical, though.

Astral:

Magic Duel against opposing Astral Dusk Elders. Solar Rays, as mentioned before. Astral Geyser is pretty cute, especially if you back it up with summoned Horrors.

Death:

Dust to Dust. Counterspamming (depending on number of Death mages you have; it at least buys you more time for your priests). Control the Dead can be interesting as a delaying tactic. Wither Bones. Naturally Undead Mastery, but outside the scope of this discussion.

Nature:

Storm of Thorns can be effective against Ethereal Undead, and also to delay the horde. I'm not sure if Polymorph works on Undead, but it's also Alt8. Conj8 has Wild Growth. Hm, maybe Nature has it worst for Battle magic answers, anyway.

Blood:

Hellfire. Presumably, Bloodletting would also be effective.

This is a quick analysis.