[Saber Cherry]

Your enemies will never fatigue, so they will eventually beat your living soldiers, no matter how superior their stats... unless... you cast Relief!

For invading Ermor, bring Endless Bags of Wine. Herald Lances and Flambeaus are nice, just for their spells, not for combat. Troopwise, favor archers over melee, but bring enough melee units to protect everyone (heaviest armor + shield; attack and damage don't matter, only protection, defense, and morale). The best offensive mages are Fire (Holy Pyre, Falling Fires), Earth (Blade Wind), and Death (Dust to Dust, Wither Bones). The best support mages are Nature (Relief - essential!, and optionally Mass Protection), Air (Wind Guide - increases precision of mages and priests, not just archers!), and Water (Mass Quickness for 2X priest banishment).

Lastly, give your Prophet (or highest-holy priest) Boots of Quickness, Rune Smasher, Lucky Coin, Amulet of Penetration, Girdle of Might or Amulet of Resilience, and have him cast Banishment along with the other priests. As was said, you need lots of priests. Think ~40 priests with 50 archers and 50 heavy infantry, all backed by a few mages, to deal with armies of 1000 undead without significant losses. You need live soldiers and mages because the AI always mixes living (starving) units with its undead hordes. Ideally, your losses will only be from province defense (30+ at a single-province chokepoint, or ~20 otherwise) when you are the defender. Wars against Ermor require more PD than normal.

Note that if you know ahead of time your opponent is Ashen/Soulgate Ermor, taking a Water-9 pretender is a good idea, just for the extra banishment.

P.S. Don't forget Fanaticism / Sermon of Courage! If your infantry run away, you lose 2000 gold of priests.