Damaged Carnage with fire at least 3 times before defeating him.

During the Carnage boss fight you can activate levers around the room to unleash 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 the boss to get close to you and hit L1 to pull them. When Carnage jumps up the wall you should activate all the levers around the room and hit him with a Seismic Blast to make him fall down into the fire.