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 5. Whoever is getting the achievement just needs to kill the other person 5 times to win.
It is also 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.