This is a five part battle.
Warp to Spree and grab mostly reds and greens, and a few browns for defense. As you head into town you will be ambushed by a couple bandits, kill them then replenish you numbers if needed. Once in town there will be a cut scene go to where the eye of beholders are and position your green and red minions on the ledges above the creature, and use your browns to defend them. This need repeating three times, then the towns people will beg for forgiveness, if going for 100% evil kill them otherwise warp back to the tower.
Once in tower warp to Heaven's Gate, there will be a cut scene with eye of beholders grab all reds and place them on a guard marker and position them above the eyes. Defend the reds with yourself from the melee bandits coming up the stairs. Repeat for each eye, once the eyes are destroyed send half the reds back to the pit and fill your ranks with greens.
Go up the stair into town and another cut scene will play. When the scene ends Kahn will attack you move yourself and you minion to the adjacent ledge. place all the minions on a guard marker and position them near the edge, they will make short work of Kahn and the eye. Once Kahn has suffered enough damage he will smash his way to the next area. As you move to the next area you will be attack by a small force of bandits, kill them replenish your numbers and bolster your minions, and empty the treasure chests.
Before entering the next area there is a cut scene with eyes spawning bandits. When you enter the next area immediately move you minions over to the drawbridge wheel and close the gate. Then kill the bandits, then run to the other ledge there is a green spawn pit here spawn more greens if needed cast a spell to increase your minion attack strength and position them on the edge at a guard marker. Move from ledge to ledge until Kahn has taken enough damage to move on to the final area. On the way to the final area you will be attacked by a small bandit group kill them replenish your numbers.
In the final area there are three eyes and Kahn. At the start run to the top most ledge kill the eye closest to you and place the minions at a guard marker at the edge. Boost you minion with magic if you can and they will quickly finish off Kahn. Once Kahn falls to his knees, cut him with your sword.