Of course, it all depends on Ermor's theme. Considering his growing army, you are probably playing against one of the two undead-only themes.
First of all: If he shreds your priests, there haven't been enough priests. Don't rely on bringing your expensive giant priests with you, but rather, hire a swarm cheap indy ones. Don't place them all in one spot. Banishment, which is the key to beat off Ermor (unless you have death magic on your own), has a very long range.
Second, Ermor usually tends to have a very strong dominion. Preaching it out of existence takes its time, and I find it especially obnoxious if the lands that feed your sieging/preaching army are barren.
Ermor gains troops for free, depending on the Dominion of the province (at least, the two undead themes do so, I am actually not sure about Broken Empire), or by reanimating them with priests. In the first case, the quality of the troops is determined by such facts like if there is a castle or temple in the province, and the reanimation process generally requires either corpses in the province or population.
For more money, you can always wish for it via Alteration 9 and lots of astral.
Oh, okay, a more feasible way of getting more money is to hunt for fire/earth sites, or forge items that produce gems for alchemization.