Haven – "Everything is mixed up, truth and falsehood" [Campaign 3 – Mission 2]

Yeah, I don't really play the previous installments or the 6th one professionally, just to pass the time. I don't know many of the nuances, so I usually try to overwhelm with numbers or attack from a distance with archers or spells. I haven't really learned how to use magic effectively. And now I'm completely stuck, I haven't encountered this before. I managed to defeat the first two barons (it seemed like I did it quickly), but the third one has such a large army that I didn't stand a chance. Then I still attacked him with smaller forces and won, although I lost almost my entire army. Now I'm just standing here, rebuilding my army, I don't see any other way. I tried starting over, but I don't feel like it. It's a shame.