Ranger "Refugees" - [5th campaign, 1st mission]

I figured out the algorithm, if you can call it that, for Biara's appearance. She also didn't appear for me for a long time. If she doesn't appear by the end of the second month, restart the game. The algorithm is as follows: she won't appear until you lose one of your outposts to the Necropolis. Then you happily watch the cutscene about needing to retake the outpost – you retake it, and after a couple of days, Biara arrives, and before the cutscene, it starts showing the turn of the red player (that's her). So, if you didn't have a cutscene about retaking the outpost, Biara won't stop by. I've repeatedly, out of simple human curiosity, started the mission with different scenarios and found the simplest and most effective one. Hire 3 additional heroes and place them between the garrisons with armies capable of defeating 50 skeleton archers (approximately). In my experience, they don't leave more than that in the garrisons. Use Fedya to run around the upgrade locations; there are some there. With another fast hero (try to develop logistics), run around collecting animals at dwellings. I captured the garrison on the third attempt. First, I completely crushed everyone. And also, after two attempts, I took creatures from the garrisons through a simple exchange. Take 7 fairies, divide them into groups of one, and move them to the garrison, replacing 10 ents. Simple and effective. After the red player's turn bar appears, all the companions rush to the Castle, led by Fedya. There, they drop all the exchanged troops and stand aside (apparently, they're taking a break). Fedya arrives, sits in the Castle, and fights Biara there. It's VERY easy to defeat her there (the Hero). Yes, I almost forgot – you need to level up your troops; capturing the fort on the hill is highly desirable. Level up Fedya's light magic, logistics, and luck – these are essential