Colonization

Question about the third mission. I developed my army, approached the enemy castle, and saw an amazing army. The hero at the very beginning clones all his warriors and destroys all my warriors in one turn. Is this some kind of joke? How do I take this castle? I'm playing on Recruit difficulty.

Accumulate an army and collect all the artifacts (when I played it myself, I killed the main character using "banishment" in summoning magic, developed defense, and light, along with a huge amount of initiative (I played on Veteran difficulty; I assume that here, as on many other maps, it's easier to pass on a higher difficulty level than on a lower one, because you get more experience for killing neutral creatures. But that's not for sure ;) ). I planned to add a Hard mode to the finale on Veteran difficulty. A hero. Which I ultimately didn't implement.