Awakening of Evil

I'm stuck on the first map.

After descending into the underground, how can I defeat all the enemies there? I only managed to defeat one.

The enemies have such strong units that they usually kill a stack in one hit, or almost.

I saw people writing about vampirism, but if I have 1000 Furies and they take a hit, and only 200 remain, it doesn't seem to help much.

Or is there some secret strategy involving Darkness here? Like Berserk/Subjugation? I remember finding some scrolls, but I sold them all, thinking I was invincible.