When I test your code it seems to me that the archers are not intentionally attacking the ice yak. They are attempting to attack the Ogre Chieftain from a distance despite the fact that there is a wall preventing the arrows from reaching their target. These arrows ricochet off the walls and attack the cows and provoke the ice yak causing the ice yak to kill the archers. You need to lure the ogre chieftain from its position to the ice yak. Then the Chieftain will provoke the ice yak, and the ice yak will defeat the Chieftain for you. Then you need to proceed and kill the witch and defeat the robot walkers.