Ice and Fire

I'm trying it on difficulty 1. I went through the passage and met Brand, level 27, with all stats around 30. I can kill him, but I'm losing my army. So, there's no point in killing him because he has a garrison, and another computer-controlled hero with a decent army and, I assume, similar stats appeared. There's nothing left to break through with. So, I'm wondering how to do it nicely.

Hmm, when I played, the first hero I encountered was in the first castle, with a small army. As it turned out, the AI was gathering all its forces in its capital to give me a warm welcome in Wolfenstein. In general, I captured the first castle without any problems. But the second one was more fun. The heroes were around level 22-25. I suspect this happened because a lot of time had passed since the beginning of the second part, and the AI had time to get stronger.