When I got this, I waited at the archway where the Shriekers were and used a ranged attack to draw Alexander to me. By Alexander's third turn, a giant void worm will appear. It will take the attention of most of the magisters. The worm isn't strong enough to fight off all of them, so let the magisters kill it and use the time it takes for them to do it killing all the melee enemies. Once Alexander dies, whether the battle ends or not, you will get the achievement.