This can be found on the Barns level of Dark Carnival. Continue until you are roughly half way through the level. Just before you go through the Farmyard, there will be a "Stache Whacker" attraction, walk up and press x and with your gun, shoot or melee the little Moustachios. Continue the game until you hit 42 points and the game will stop and the achievement will pop up at 42 points. Here is a video to help:
Credit to Rooster Teeth for the video.