Ice and Fire

I'm trying it out for the first time. I went through the map and encountered Brand, level 27, with very good stats. I can kill him, but it depletes 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 probably similar stats appeared, and I don't have enough troops left to defeat them. So, I can't capture the castle. I'm trying to figure out how to do this effectively.