CQC is the Arena mode. 8 players, Deathmatch, Team Deathmatch, Capture the Flag. All of them count. Obviously Deathmatch will be the hardest to boost, but it still counts toward level 50.
If you have less than 8 players, you'll be paired with randoms, and this game mode isn't dead.
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.
Best of luck!