[Ironhawk]

A solid way to defeat Ermor is to play the nation most diametrically opposed to them: Marignon. First and foremost, Marignon has dominion which kills undead. Second, it has a wide selection of priests (which you should be using in abundance). And Lastly its national spell is Holy Pyre so right out of the box your fire mages should be able to massacare the undead.

In strategic terms you should also attempt to go to war with Ermor as soon as possible. Ermor gets most of its base troops for free and does not pay upkeep nor have supply restrictions. Attack them while your priests and fire mages have the relative advantage.