There are two spells that will turn a battle for Ermor, even if they are facing huge amounts of priests or anti-undead spells (Wither Bones, Dust to Dust).

Darkness and Rigor Mortis. Both of those spells affect anyone not undead (which are usually the ones trying to 'kill Ermor' because they are afraid). once you have those two spells in your arsenal, your attrition isn't quite as bad (with Darkness) and you can kill enemy mages by having backrow casters with Rigor Mortis they will eventually get to 200+ fatigue and die.

But even as there are those for Ermor there are other ones which will do masses of damage to them. But Darkness or Rigor Mortis is usually able to research one or the other by turn 20 to use in tough battles (as most players will mass one big force to handle 400+ armies).