in the Will of Asha campain in the Grim Crusade quest, I got as far as defeating the demon lords, but I got to the castle and the castle had an insanely difficult army... it had like 35 archdemons, 55 pit lords, etc. is there any easy way of doing this?

yes it is a tough one. I went cheap here and made Arantir use either puppet master, blindness or frenzy. Puppet master is the best if you have it, because you just grab a stack of enemy critters and use them. But it feels cheap. Blindness only works until you attack that stack, but it does disable the enemy stack. Frenzy is good too.

May I ask on which difficulty level you were playing? I assume you play on heroic..because on this level you have exactly 35 archlords, 55 pit spawns, 12 titans, 220 cerberi.
Does the difficulty level affect these numbers (i.e. on easy are there less archlords and pit spawns?)