Gangster Brawler Achievement
Won 10 Gangster Brawls or Team Gangster Brawls on Xbox LIVE
You can boost this easy in party mode. For information on how to get to party mode, see "Party Time." Go into settings, and change the maximum kills to 10,
On a small map, this will take 2 or 3 minutes at the most. Beat your boosting partner 10 times and this will unlock.