In the second chapter of the Dark Carnival campaign you will find a shooting range. Play this game until you score 750 points and you will receive Gnome Chomsky. Have one survivor escape the rest of the campaign with him to unlock the Achievement.
Second part of the Parish campaign
Once you are off the boat head up the ramp and you'll see a barricade with barbed-wire at the top. Jump up on top of that and you'll see a gib sign right behind you. Face away from the sign looking down at the water, jump forward just a little bit and it will force you to hang from the sign instead of the barricade over top of the water. Someone will help you up and you will be on top of the sign.