Okay, here's my 2 cents on this achievement. You need to complete all 12 waves of Beast on the following new maps:
5 maps in all. Note that I've only played 3 of the maps so far (Aftermath, Artillery, and Cove) by myself, and they are a bit massive. I'd recommend playing with a friend or three so you're not stuck trying to find all of the humans to kill and the timer runs out on you.
UPDATE: I've completed all the maps, thanks to some help. All but one of the maps are massive, the exception being Raven Down. However, the higher you go, the more obstacles and enemies are grouped together so you might be overwhelmed if you play by yourself.
If negative feedback is given, please leave a comment so I can improve this solution.