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?
There should be a counter for the monster zoo under your minimap, showing how many critters are left. The counter goes away when you complete killing the zoo. That said there is (at least) one bug associated with monster zoos that can screw up the counter -- for example if there turns out to be more than one zoo on that level (which is not actually supposed to happen). Hopefully that will be fixed shortly. I'm not sure if that's what has been causing your problem or if there is something else going on.
Hmmm... I never noticed the counter before and I've never run into a 2nd zoo on a level. I'll keep my eyes open the next time I run into a zoo and see what happens.
If you run too far away from the zoo or leave the current floor and come back, it will bug out and the Zoo Counter will either freeze at whatever number it is currently at and never update, or simply disappear altogether. So really, the only way to get the achievement currently is to have a chokepoint nearby, and a bit of luck.
Yeah, there's a huge list of things and conditions under which this won't trigger: multiple zoos, saving, loading, going upstairs, going downstairs, and using it in months ending with "y". We're on it, but probably not for this patch.
