Problem with Haven.

The leveling system is standard there – the opponent has a hero focused on strength (attack + defense, and possibly "inspiration"), so their troops are tougher.
But the Warlock is strong because of his magic.

On large maps, it's quite easy to find good artifacts against Chaos and level up perks. The Warlock really needs education. You could have played it smarter, but you shouldn't drag it out. Although, 15 Knights and 12 Dark Knights shouldn't take that long.
Did the Warlock have defense and education?