Choose, TDM, Team Death Match, under Quickmatch. This has to be done in one TDM game, but its pretty simple as the teams are divided in a 6v6. You will have to kill 6 unique players in one match. If it's a full game (6 vs. 6), you will have to kill every player on the other team at least once. If a player you have killed drops out of the game, the kill will still count. If another player joins, and you kill them. It will count as a unique kill. Once you have killed 6 different players, the achievement will unlock.
Click HERE to find partners to play with online or to create boosting sessions.