I know a lot of gamers go on about the 2 guys mid way through chapter 4, but this can be done earlier and with a lot less hassle. In chapter 3 after the submarine and when you re-meet Elena, you'll be faced with a solo guard who conveniently has his back to you. Melee him from behind then either die or restart checkpoint (which is literally 5 seconds back in the game). Each kill takes about 10 seconds, so farm the trophy here and you'll have it in no time.