Haven vs. Necropolis

I've always questioned the usefulness of the Agony skill.

When the final battle is between heroes of level 11-14, there's plenty of mana but not much health left – why not? Agony lasts for three turns, slowly dealing damage. In that battle, I regretted not having mass Agony and some kind of Lightning spell. I lost, but I still had a lot of mana left at the end. The Knight ran out of mana on turn 2.