For this, you have to win the following: Stronghold, Alpha Lair, Outpost, Contact, Quarantine and Research Lab.
All of these can be done with 2 players. Either grab a boosting buddy or play with a 2nd console and extra copy of the game. If you're using a 2nd box, your other account will eventually drop from inactivity, but the win still counts.
For Contact, Quarantine and Research Lab, the match goes faster if your main account is a Snow Pirate since that team can run around and complete objectives (which will net some other achievements too.) if you're on the NEVEC team, you have to wait out the time since you're trying to prevent the objectives from being complete.
At this time (10/13) there aren't many players in multiplayer.
Thanks to BigPapaBee for adding Stronghold to the list of games.