AI in Might & Magic: Heroes VI

I've encountered a problem: while playing Heroes 6, I tested all the factions and all the heroes, but I constantly face a very frustrating issue – namely, the inevitable defeat of my main hero by the AI's main hero! Let me explain: when the game is nearing its end (meaning when the entire map is revealed and you're simply trying to catch each other), I have a hero, let's say, at level 25, while my opponent often has a hero a couple of levels weaker than mine. However, when we clash in battle, it turns out that he's a mage, and I'm a warrior, so our stats are different. I have 35 attack power, but I deal ridiculously low damage to his units. In turn, the AI hero, with only 10 attack power, deals incredibly high damage to me, damage that I can only dream of dealing even in a battle against neutral creatures! Does anyone know how to deal with this? Please advise!