[K]

In my experience, the Level 1 priests you can recruit are almost useless for this purpose. This is because Banishment's AoE and damage is based heavily on Holy level, and Holy 1 makes a very small Area of Effect and doesn't do much damage.

Holy 2, however, is the real deal. Put five Holy 2's on the field and you will see more killed undead than ten Holy 1s.

Also, when fighting Ermor and blasting them with Banishments, you want to make the combat go longer, so they spend more time dying. Simple infantry recruited out of a Indie province is BS. Put heavily armored and hopefully Lifeless things on the front (I like Statues), but regenerating meatshields work as well. Luck armies are also ideal.

Stuck with Holy 1's? There are several ways to increase Holy level. They are Prophet(one H3), Crystal Shield(E1,S1 for for +1H) or Power of the Spheres(S1 spell and one pearl for +1H), Communions or Sabbaths(easy to get a +2H with four slaves), two artifacts(+1 Hs), Hellpower(B3 and 3 slaves for +2H, beware the Horrors), and summoning Telestic Animations(H2).

Done correctly, you should be throwing down so many kills that your priests will be competing against each other for spots on the Hall of Fame, and you have losses per battle in the single digits. Ermor's real weakness is that you basically know the 6-7 kinds of units it'll use, and you'll see SCs and some summoned crap thrown in for flavor.

Ps. Penetration rules. Eyes of the Void and Spell Focuses are a must to counter a self-buffing Ermor.