R.U.S.E.

you can either boost this achievement or try to get it online. If you want to boost this add 3 boosting partners to your friend list and play a game together( ranked 2vs 2 or normal) You can also get this by joining a session and add the other 3 people to your friend list, and have them confirm it before you join back in. (thanks for comment)

Edit:
Just to make sure, private matches work perfectly. Choose a 2vs2 or a 4 player FFA map and invite your friends. If one of your friends hasnt downloaded the free maps make sure you choose a map that was included in the main game. Once the game has started the achievement should pop along with the "play a game with a friend" achievement if you didnt have that one.