Sylvan vs. Necropolis

With a morale of -8, the main thing is to root the ancient Ents (they have negative morale, and as far as I know, it doesn't happen often). And that's it; the Necropolis creatures won't be able to do anything anymore.

Added 1 minute ago
This is, of course, just a joke, but in this case, I would level up Darkness and cast Vampirism. After that, the warriors won't care about morale.