Haven – “Defeat and Victory” [Campaign 3 – Mission 4]

This is all very instructive and useful, but... I'm playing on easy mode because I don't want to get bogged down; I'm playing for the story. I was playing calmly, without any stress, albeit slowly. And I got stuck on this map. The problem is that the red faction sends a hero to attack me who uses the "fireball" spell (or something like that, which hits an area) and wipes out half of my army!!! I'm fed up with it! He has a small army himself, and the game says he poses a low threat, but in battle, his first magical attack levels the playing field))). I've beaten him five times, and while I'm recovering from these battles, he comes back again. As a result, he has evened out our chances, and now I can't defeat him with the remnants of my army, and, as I already said, I can't gather a new army in time because he won't wait for me! What the heck is this, why does this magic deal so much damage, what should I do??? Help me out, guys!