Well, I think I described the steps in detail above, so it seems your Agrael is not leveled up correctly. I can only suggest replaying the entire campaign as the Dungeon faction and leveling up Agrael properly. Here, for example, is my Agrael at the beginning of this mission .

My Agrael is at a lower level, but has a couple more skills...
Yesterday, I captured a Dungeon town, defeated the dragons, and made my way to the final town. If I had ranged heroes, I would have won, but as it was, I had to watch my army get destroyed and wait for the stupid catapult to finally destroy the towers and gates. I was just a little short :) I think I'll try to capture an Elven town and recruit some archers. And then back to the Dungeon and "gardens, gardens..."