Ermor?

[Agrajag]

magnate said:

Er, don't you mean Wither Bones? Bone Grinding damages *all* units on the battlefield, normal and undead alike, including your own. Wither Bones is undead-specific.

CC

Let me rephrase my misguiding quote:
Bone Grinding auto kills all skeletal undead because they are made purely from bones.
I'm not 100% sure its true, but 300 Longdead Horsemen and some more misc. Longdead troops say otherwise.

So with one spellcast, you destroy all of the undead.

A good spellcaster for that is the Lich, you can easily give him the requiered Death paths and he is immortal, so he can attack the army, grind, and escape, with no risk at all (under friendly dominion) and with a great chance of destroying an entire (or most of) undead legion.