The best chapter to get this achievement is on Episode 1, Chapter 4. I highly recommend making a custom loadout with prometheus vision for this achievement. After completing the first mini objective (activating the switch) you start going back to where you came from and a white knight will spawn around the corner. You will be informed by a voiceover. At this point, crouch around the corner and use prometheus vision to watch the knight walk back and forth, as he turns his back towards you, run up to him, and hold RB to preform an assassination, unlocking the achievement.