open co-op special operations go to operation strong box. (make sure you select ammo boxes for munitions), start game get to bank start sawing the door open then stop , kill then endless horde, scavaging the building and surroundings for ammo boxes , unless you have moron group member that opens door it up in 2 mins... lol ull get your weapons level to like 50 in a hour or two if you have a group wanting to do it this way