[Cainehill]

A number of other spells are very effective against the petty undead hordes as well, presuming you have the proper mages. For example, Abysia can easily destroy huge armies of normal undead using spells like Falling Fires, never bothering to cast Holy Pyre or Banishment. Bladewind is also pretty good, especially against Soul Gate's ethereal but fragile minions.

And of course, one thing no one's mentioned here is the need for nature magic, as you really need to get wine skins or other supply producing items to go very far into Ermor's dominion (unless you're using summoned troops with no need to eat).