To earn this trophy, you must use the arena interaction in Slaughter Swamp to finish off your opponent. Note that this requires a strong character, so pick anyone from Bane, Superman, Wonder Woman, Swamp Thing and the like.
Set up a Versus or Single Fight on Very Easy, pick one of the aforementioned characters and any opponent, then chip away their health until it is very low. Then, a little to the left of the arena, you should see a crocodile hanging on a couple of ropes. Make sure the
button pops up near your health meter, then simply press
to swing the croc and, if connected, finish your opponent off. If it is used before you can finish them, you will need to restart the match and keep your opponent away from them. Maybe pick a smaller foe, such as Robin, so they can't ruin your chance.