A year later, I decided to play Heroes 5 again. At first, I also got stuck in this mission. Then I remembered a trick:
The hero has the Support skill. I apply Support to the centaurs. When they attack a target, the hero also attacks. So, whether it's a ballista or a catapult, the hero's attack destroys it in one hit.
I sit in the castle and, with the first attack through the centaurs, destroy the siege engine. While the enemy breaks through the wall with cyclops, I can damage his troops.
Then the purple hero flees, even though he had more troops during the castle siege.