Play a match with a strong, muscular character (for example, Cyborg or Gorilla Grodd) on the Slaughter Swamp map. At the end of the match, when your opponent has very little health remaining, grab the crocodile that is hanging in the background (press R1) to swing the crocodile at your opponent and land the final blow to end the match and get the "Killer Croc" trophy. Characters with a lean build are not strong enough to grab the crocodile -- so it has to be a strong fighter like Cyborg.