Hooray!!! I finally completed it! And I didn't waste time on the demon castles because they had really strong troops, and I would have ended up with nothing after winning. The main thing is that I killed the mother of nightmares, and then the duke. I didn't collect 200 Kirins; 80 was enough. When I was dealing with the mother of nightmares, I positioned all my troops behind her and attacked her from the rear – she didn't even turn around! In general, it was almost without losses; she killed a few archers, but I resurrected them.
I realized that the main task is to wait and build up a good army, and then everything will be okay.