Kill someone with a fish.

Throughout the story during the brawling sequences, you may notice large fish tanks which you can perform an environmental kill on. Once these tanks are broken, fishes will drop out and flop about on the ground. For obvious reasons for this trophy, simply pick the fish up, and start wailing at a nearby hostile triad member until theyre dead. Kill them before the fish is deemed unusable, and the trophy will unlock.

Replay the story mission, Club Bam Bam or "Final Kill" if you dont have this by the end of your playthrough.