:) I downloaded the save file for the battle and defeated this hero; he couldn't even escape. It took 5 attempts, but I finally managed to defeat him with almost no losses. I only lost the horses.
Either something glitched, or I don't know what happened. Anyway, the battle turned out like this.
I reloaded a couple of times; I need to make sure the mages cast a slowing spell on the enemy.
Then, we cast Berserk. The dragon takes down his warriors, then our horses. Then, we resurrect the horses with the Archangel. Then, something strange happened. He cast antimagic on the dragon, I placed the griffins in a paid position, and cast Berserk on two of them. The dragons, with 1500 health, just stood there and skipped their turns.)))
Here's the save file after the battle, showing how I defeated him :).

Send me the map later, and I'll try to complete it.