There are 2 popular methods suggested to get this achievement, the first is to use the knife to stealth kill opponents the second uses the RPG.
To get it using the knife your best bet is to sneak up behind people with the knife out and hit RT as soon as the indicator pops up. this can be tricky at times, but it is do-able. be sure not to miss the Prompt, or your character will lunge the knife feebly through the air, forcing you to start again on a new match.
Some say the RPG makes things easier. The trick here is to use the lock-on, which can be found by pressing the RB button once the RPG is equipped. then, aim at your target and wait for the lock on to complete(illustrated by the cross-hairs converging and turning red). Once this happens, and you are sure you have a clear shot, press fire and let the rocket do the rest. Now move onto your next target! The best map for RPGs I believe is 'An Invasion'. It has a few of them scattered around and the map is open enough to allow the rockets to fly.
Whichever approach you choose, be sure to select a team playlist (preferably a large team playlist). reason being, matches will take longer, allowing you to have more chances to run up behind people and stab them in the back or more opportunities to get a good lock on.