gotica2905, it's actually quite simple, but not fast))). I had to put in some effort. Here's how I did it:
1. Captured Ashwick in the third week.
2. Got the Fishing Village in the fourth week, but the red faction successfully took it from me. In the end, I was left with two Castles.
3. Gathered a decent army and defeated the Queen (it was quite easy because she's slow; I flanked her with Shark Guards, Kappa, and Kensei, while leaving Snow Maidens and Coral Priests in the front; she only attacked the Maidens, but they didn't die quickly and held their ground; the Shark Guards, with the help of Kensei and Kappa, defeated her in five hits).
4. Immediately after defeating the Queen, the Demon Lord, with his "deadly" army, decided to capture Ashwick. Irina quickly teleported from the dungeon to Ashwick and repelled his attack (I honestly thought I would lose to him).
5. About two weeks later, I captured the Demon Lord's Castle. By that time, the demons had captured all the Castles of the Duchy, except for the Castle where Mark is located.
After that, there were no more problems because the demons didn't rush to use the light troops of the Alliance. Irina quickly captured the remaining Castles and defeated Gerhard.
By the way, is it supposed to be that Irina is attacked by "minor" and "low-level" armies of the Demon Legions and Gerhard's Castle from the first week?