This almost certainly needs to be boosted. People are almost never online, and most legit ranked games are far too small to get this achievement.
Set up or find a session and have at least eight people (including yourself) take turns winning a ranked deathmatch (turn off respawns so the game goes faster). Have the person who is going to win use a distinct character (red armband, white coat) so everyone is aware of who will be winning.
Play smart, and everyone can get this in 20-25 minutes.
***update*** As xMidget Furyx commented below, you can set the map to Lab Rat (a really small map), and set the score cap to two kills.