In Chapter 2, search for a single enemy near a checkpoint, then sneak kill him with your knife from behind, and load the last autosave. Repeat this until you have performed five sneak kills without being detected to get the "Silent Kill" trophy. There is a great spot in Chapter 2 where an enemy is standing next to a campfire, and there is a checkpoint at the same place. Kill him, pause the game, load the autosave, and repeat. You can also farm all other kill trophies in the same way.