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

Maybe I'm a complete newbie, but I've tried all the tactics suggested in the thread on this map. It didn't help. I just don't have enough troops; my hero is a strength-based hero, and all the upgrades are appropriate. I'm currently playing this map on easy; I couldn't manage it on normal. Even with the most "loss-free" battles possible, I captured the starting castle with the hero's spirit, the castle underground, the castle right after the river (which is neutral), and two purple castles. I approached the third one (which is in the north, in the snow), defeated the hero (not Igor), and Igor became deadly for me. So, I thought I'd gather troops for a couple of weeks and then give him a good beating. But after 2 weeks, three (!!!) deadly heroes came to me. What can I say, it's beautiful. This is the best outcome I could achieve.

Either the map has some evil scripts, or it's some kind of progression block, saying that you need to do something else first, or I'm just a noob. Or maybe the developers made some poorly thought-out changes to the balance in patch 1.5.2. I don't know...