If you don't feel like getting this achievement through normal play I recommend that you use the strategy which I am about to tell you.
The best way for you to do this is to set up a ranked team elimination game with rifles as the base weapon, the battle gauge set to 10,000 and a 10 minute time limit. The map you choose is entirely up to you, I suggest you go with the map you know best.
Then you get at least one other friend who owns the game to search for a match with the exact same conditions as the match you have just created. When your friend sees the game you have created have them join it, once you friend joins start the match immediately to prevent strangers from joining your game.
Once the match has started have your friend meet up with you and allow you to head shot them. By doing this you will gain about 10 head shots per game which will allow you to get this achievement after 5 matches. This will gain you 5 team elimination medals helping towards the achievement Elimination medal.
This should also gain you the chain killer achievement.