... Get the Horns
It is worth 25 Gamerpoints.
This achievement is represented by a bronze and silver signpost with a bull head on top of it.
This can be done on Rumble Pit if Braaains on Dismemberment is selected, because zombies have swords, and humans have shotguns, so it's very easy if you are lucky enough to find Mythic Infection. It is also easy to do with Grifball on Grifbox in the Action Sack playlist.