Mid/late game defense against undead??

[Taqwus]

Flames from the Sky would be nice, since its effect scales with target army, but it's not exactly the easiest research. Battle spells that hit large areas or the entire battlefield can also help. Try to have your own necromancers, as well -- Wither Bones is unresistable.
Archers with Flaming Arrows activated could help. And a War Bow fires 13 arrows per turn... might be nice if you have a quick/precise commander.
Try to have an unbreakable, unfatiguable front line -- this usually means mindless summons such as living statues, clockwork horrors, et al. The priests and mages must be protected... at least there aren't that many Ermorian/necromantic fliers (Harvester of Sorrows, Lammashastas come to mind).