RussianScorpion
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I'll tell you how I completed this mission on the Hero difficulty level.
Russian version + patch 1.1
In general, there was only one problem, and her name was Biara.
At the beginning, I developed peacefully until I captured the first border outpost (in my case, it was the second one, looking from left to right). After that, I moved my main hero to the left of the Castle, captured mines and stat-boosting locations (at that time, I didn't know that losing a border outpost was unacceptable; I thought I needed to lose them all), but I simply went down and retook my outpost. After that, they captured my 5th outpost, and then, after some time, Biara appeared with an incredibly strong army. After capturing my second outpost, I returned to the Castle, took all the troops, except for a few Dancers (I took several of them) and Fairies, and this is where the climax of the playthrough begins. With my main hero, I run to the middle outpost, and with the other hero, to the 4th outpost. With my main hero, I take all the troops there (they can't be transferred, but you can exchange them for some of your own, so I exchanged them for one Dancer per slot) and run to the 4th outpost, where the second hero does the same, taking the army there. Slightly above the 4th outpost, there is a creature upgrade location, which I used, getting a total of 4 creature slots (without Fairies and Dancers).
After Biara appears, the main quest of guarding the border is marked as completed (you can now ignore the outposts), and a new quest appears: to destroy the demons. After upgrading the army, we run towards the Castle (Biara is also running there), and I tried both defending in the Castle and simply catching up with her on the way (at that time, you could still visit a stat-boosting location along the way), and that's it; with such an army, it's easy to defeat Biara.
Russian version + patch 1.1
In general, there was only one problem, and her name was Biara.
At the beginning, I developed peacefully until I captured the first border outpost (in my case, it was the second one, looking from left to right). After that, I moved my main hero to the left of the Castle, captured mines and stat-boosting locations (at that time, I didn't know that losing a border outpost was unacceptable; I thought I needed to lose them all), but I simply went down and retook my outpost. After that, they captured my 5th outpost, and then, after some time, Biara appeared with an incredibly strong army. After capturing my second outpost, I returned to the Castle, took all the troops, except for a few Dancers (I took several of them) and Fairies, and this is where the climax of the playthrough begins. With my main hero, I run to the middle outpost, and with the other hero, to the 4th outpost. With my main hero, I take all the troops there (they can't be transferred, but you can exchange them for some of your own, so I exchanged them for one Dancer per slot) and run to the 4th outpost, where the second hero does the same, taking the army there. Slightly above the 4th outpost, there is a creature upgrade location, which I used, getting a total of 4 creature slots (without Fairies and Dancers).
After Biara appears, the main quest of guarding the border is marked as completed (you can now ignore the outposts), and a new quest appears: to destroy the demons. After upgrading the army, we run towards the Castle (Biara is also running there), and I tried both defending in the Castle and simply catching up with her on the way (at that time, you could still visit a stat-boosting location along the way), and that's it; with such an army, it's easy to defeat Biara.