Shouldn't take more than a minute. First things first, you obviously need the DLC pack "Medieval Horror" downloaded and ready. Then you need to grab a boosting buddy or group (Feel free to message me "FFX Brotherhood").
You then need to set up a "Death-match" game-mode on the map "Psycho" which is essentially an old and run down asylum. Set the score limit to 1 and the timer to anything about 5 minutes (Just to make sure you can find each other). Then, the person going for the achievement simply needs to kill the boosting buddy. Since the score is set to 1, the match will end and you will get the achievement. You can then swap so your partner gets the achievement.
This does have to be done in ranked, however this game is nice in that it will let you invite your buddies into a game. Since the multiplayer is normally dead and even less people have the DLC, you shouldn't have any trouble with running into randoms.
Good luck, and enjoy your 50G.