Find a good spot with a few kills. There's one at the end of Chapter 7 when you re-enter the ballroom. On my second play through I was getting 12-15 kills, and then restarting from checkpoint. I did that about 8 times and got the trophy.
You CAN die outside of combat (according to others that doesn't reset this). There is a few places with "infinite" spawns in the game.
Another location is the beginning of the game, you can ram a few other boats, and then restart checkpoint and farm the kills that way.