During the Carnage Boss fight in the "Maximum Carnage" mission, you can activate levers around the room to release a burst of fire near them. This must be done in the second half of the Boss fight, after Cletus Kasady turns into Carnage. Simply stand under the levers, wait for Carnage to get close to you, and press LB to pull them. When Carnage jumps up the wall, activate all the levers around the room and hit him with a Seismic Blast to make him fall down into the fire. Do this three times to get the "Friendly Fire" achievement.