The grenade camper
The grenade camper buys the Kar98k at the very beginning of round 1, and doesn't use it until later rounds (i.e. round 3) and saves up for the Help room. He buys the mystery box once and no more until leter on. By now it may be round 5, and the grenade camper buys the upstairs area found in the help room (NOT THE STARTING ROOM), gets the BAR, and goes to the last upstairs room where the wreckage may be removed to unblock another way downstairs (which he will not unblock). The grenade camper continuessly buys grenades and BAR ammo (and maybe the mystery box) to sustain his ammo while camping in the corner near the grenades. This strategy proves more effective in two player as the player with the weakest weapons can guard the window to the right of the original grenade camper.