This is a CQC achievement. Normal CTF rules, two teams, 3 flag caps. You can't score while your flag is being held. If you have less than 8 players, you'll be paired with randoms, and this game mode isn't dead. The best way, if you're able, is to boost this on the way to your 250 CQC wins and your level 50 achievements.
Please note that 2 teams of 4 can't match together. The best way is to have a group of 4, and try to have the other 4 join search for the lobby. However, once you have a lobby, you'll be in the same lobby after every game, so you can alternate wins. (Of course, for this one you only need to win for the first time.)
Best of luck!
