Before the fight, move all of your units to the Amina, then trigger the script starting the battle using Salaar. When you lose, instead of "game over" it still be your turn, allowing you to continue. Wait a turn or two for "hero" Ferre to get out of the way, and clean up the undead units summoned in cut-scene. Now you are controlling the Amina, except she is invisible. She still has the units you gave her (You also have Myzrael, but he is weak and almost dead anyway). Move forward on the road below the castle until you stumble upon invisible "boss" Ferre. Killing him will trigger the proper end of the mission.

This is easier if you level up the Amina with gold in the previous mission, and give her all of the prime hero's equipment, since it'll disappear anyway. The main army should be enough to kill Ferre though (especially units immune to physical damage).

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