I've killed 3 different monster zoos on 3 different occasions and I've never received this achievement. Is there a requirement that I don't know about. Usually when I run into a monster zoo I retreat to a choke point and then just slaughter the monsters as they walk up. Once the crowd thins out I move forward and mop up. It is possible that I missed a monster but on 2 of these occasions there was no place else for them to go (no side passages). What gives?