The best place to get this Achievement is in Chapter 2 after you learn about the Sneak Kill you're going to come to an area with 6 enemies simply crouch walk up behind them and press
when prompted.
1.Hide behind the hay after the cut-scene and wait for the enemy to walk away crouch walk after him and kill him.
2. Hide up against the corner and wait until the enemy is walking away and follow him and kill.
3. There are 2 enemies up ahead but you don't want the one on the road first, take the other guy out by sneaking around the hay stacks.
4. Take out the guy on the road.
5. Sneak through the building disarm the trap and make your way outside for no.5.
Getting this Achievement will cost you the Achievement for not killing an enemy on Chapter 2 but you can reload to previous saves.
*for some reason the Achievement will not trigger for me but I used another account to get it and my video shows exactly how to do it*