"Escape to Salvation" - "Magnificent Retreat" [Campaign 4, Mission 4]

I'll tell you how I completed the battle with Biara. Zephyr had Arcane Knowledge. I leveled up Plate's Elemental Fury, education (Scholar), and Chaos. Zephyr learned Armageddon. By the way, there's an unclear bug here – messages like "Plate receives 'Call to Arms'" appear. But the spell doesn't appear in his spellbook. It would be interesting if it did. I take gargoyles for the alien – they're not needed by Zephyr anyway, and 6 gremlins. The gargoyles stay in one slot, and the gremlins – 1 in each. I move to the castle (the enemy's), move through "reinforcements," and attack. As soon as the hero's turn comes, I use enhanced Armageddon. And I do this several times (with each summoning of thunderbirds, the alien appears with the same stats). In one week, I killed 180 demonesses, 10 devils (in one stack), etc. In general, in the 3rd month, Zephyr easily takes the castle.