This should come over the course of playing towards other trophies. However, if like me, you wanted this out of the way I found after one playthrough of the game, loading up Chapter 11 - Deadline was quickest for me. I used the Tony mask (unlocked by getting High Score on Chapter 1) as it is a one hit melee kill on most enemies. As you enter, kill the guard with the silenced pistol and you will be rushed by 4 melee enemies, try to take all these out with punches, then if successful, try to take out one guard to the right with a gun. You will be taken out by another guard with a gun more than likely but don't worry as death does not reset the kill counter and you can keep using this method to rack up the kills.
EDIT: I played on the Vita only, and as TheNegativeIon mentions in the comments below, it may not work on the console version. Any confirmation either way would be appreciated.