How do I defeat Orwald in the “Blood of the Dragonfather” mission?

Good day! Yesterday, I started playing this mission, and when I exit the dungeon, Orwald has 15 black dragons and 20 red dragons, along with hordes of other troops. I only have 24 black dragons, and my army is about half the size of his. How can I defeat him? I didn't invest much in Mutare's secondary skills...