Well, I replayed this damn mission on Recruit difficulty. This time, Biara appeared on the 3rd day of the 2nd week of the 3rd month, as she should. During this time, I leveled up a secondary hero. I was lucky; Letos appeared in the tavern! I maxed out his Chaos magic to Expert, Expert Unstoppable Magic, and Arcane Wave. I also bought the Robe of Sar-Iss from the artifact shop, and before the battle with Biara, I visited the Tower of Mages and learned Armageddon and Explosion. By that point, Letos had 22 in Spell Power. With 22 in Spell Power, I only got Rai'lag in the last mission for the Warlock, but here, in the first mission, I have this beast, Letos! :D Anyway, I put Uncle Faidan in the Castle and went with Letos to beat up Biara.

Added 3 minutes ago

It's quite simple to lure her into your territory by drawing the troops away, and then trap her at the crossroads.

But she just stood there like a statue and wouldn't move, the pest! I even put Faidan and his army in the Castle, but it didn't help