Well since no one else has said it, after completing Lexington and Concord in sequence 7, you should find yourself back in the frontier with your next main mission icon all the way to the east of the great pierce hills region by a viewpoint. As you head towards it, a convoy will spawn and you are tasked with intercepting it.
I switched to fists and knocked everyone out and the achievement popped right away. This also served as the database entry for the boston brawlers club.
Of course this was after I spent an hour running around trying to get one to spawn