Shown in the video @8:30 "Sorry for Party Rocking" for 10 GS
For this achievement you will have to Destroy an infestation and you will unlock the achievement. Definition
Infestations are a group of zombies residing permanently in a building marked on your minimap as a red house after discovery. The group of zombies usually consist of 8 or more zombies including 1 or 2 screamers. Strategie
Explosives are a great strategie, just open the door and throw them in. You car is a very safe way of disposing of them as well. Open the door get back in your car and just drive over them as they come out. No matter what strategie you are using make sure you always take the screamers our first!
If you do not they will attract more zombies with there screams.
Finding an infestation
Infestations can be located by surveying an area or by driving/walking past an infested building. They can also be part of search and destroy missions.