Halo: The Master Chief Collection - Halo 2 - Silent but Deadly - Halo 2: Kill 7 opponents from behind in a row without being spotted. This achievement is fairly easy to unlock.
On the mission Regret, towards the end of the mission right after the second elevator ride.
BEFORE it if you kill all of the enemies in the area with the scarab gun the sword elite wont come into the room for your first assassination. After the elevator proceed to assassinate the two elites and finish off the grunts that are sleeping.
After the final assassination your achievement should pop! Enjoy!