From the main menu select multiplayer.
From there there will be a Playlist called "fireteam: dirty bomb"
You'll want to select that game mode. It consists of 10 fire teams of 4 players each team. To get a team wipe medal you must down and kill the final player alive on a single squad(there are multiple squads-beware)
To check how many squad wipe medals you've earned go to
Tab to barracks
Scroll to medals
Tab to game modes
Medal will appear as "squad wipe"
Definition: eliminate the last enemy player alive on a fire team consisting of 3 or more players.
At the time I am writing this guide I am at 4 so far.