How do I fight ermor?

[Taqwus]

Find a druid province or other area giving nature magic. Get to nature 3 preferably (nature 2 + Thistle Mace will do), and Construction 6. A Cauldron of Broth will feed 50 men from a single misc slot.
Repopulating is basically not going to happen unless you abuse Wishing for population in that province.
Medallion of Vengeance and all those assassins will be expensive; plus it's hard to assassinate a Dusk Elder because they'll summon undead like crazy, and in addition depending on how long the game is there may be Wraith Crowns which create more undead.