You need to proceed as follows: Mages appear at the very beginning of the mission, and you can't fight them (according to the plot). When you get close to them, you need to run away as far as possible, as if to break away from the pursuit. The mages move along their route, so you need to wait for the right moment and run to the right, entering the dungeon. Of course, you need to collect all the undead on the map and fight the living monsters to increase the number of skeletons in your army. When you run past the mages and enter the dungeon, the mages will still follow you, so you can't hesitate and keep moving forward. You exit the dungeon and kill the hero near the castle, and that's it. The mission is over.