To obtain this achievement you must assassin 5 people from behind in Matchmaking.
This achievement is actually easy and is cumulative so you don't have to do all 5 in 1 match. I tried this with the sword and killed 6 people hitting in the back so sword doesn't work.
Also while doing this I was being sneaky and going behind people while they were firing off in a direction. The best people to do this to are people with scopes but mainly Snipers because they cant really see anything while scoped.
A good map to go for this and is easy to get some people was BR's on Lockdown (or lockout anniversarry) in the image below is the tower where people will go on top of and fire down at others, if you sneak behind people up there you can get some easy assassinations.https://pbs.twimg.com/media/BwLZBBACIAABUw5.png:large
(not where the guy is standing, the tower in the background)
The basic idea is to hide near a good viewpoint and people will fire down or across the map at other players. Its wise to kneel and walk near them so you don't appear on their radar, this is risky but worth it because you don't want them turning around ruining your chance.
Also cannot be done in custom matches.