Rock of Grimheim

I managed to survive 3 or 4 attacks from the Imperials without any losses. I stopped at Jessica and Godric. The new Angels they have are incredibly powerful. I didn't have time to cast any spells; the Angel hero already wiped out my entire army. 50 Angels killed all 680 of my vampires in one hit. Subordination doesn't work on them, Berserk is countered by Vampirism, they reach me in two turns, and with high morale, they move even faster than my troops, even though I have a staff and a ring of speed. I have no idea how to beat them. I can't stop them with summons either.
Are there any tips on how to get past them, and how many more attacks are there after this one?