To get this you will need to finish a game of multiplayer with 8 people in it. This is harder than it sounds. The multiplayer setup is terrible, the servers are really bad, it took us 4 or 5 tries to finish it because people kept getting dropped from the game.
1. Set up a boosting party via x360a's awesome sauce site.
2. Create a private multiplayer room. Make sure to set the max players to 8 and turn down the time setting to whatever is shortest (5 minutes).
3. Invite everyone in and once they have readied up start the game.
4. Sit around (no need to fish) for 5 minutes and if everyone is still in the room when the season ends....achievement!