It is extremely unlikely that you will ever find anyone playing this game online anymore, so find yourself a boosting partner (this can be done with just 2 people) and host a Ranked Deathmatch and set the kill limit to 15. Whoever is getting the achievement just needs to kill the other person 15 times in a row to win.
It is very easy to find one another because when you've been killed by someone they show up on your HUD with a big yellow star. Also, does someone want to explain to me why you need 15 kills but the achievement picture have 20 marks on it? lol