First you need is 15,000 score (if allready found than never mind) buy the first than the second door. Then you come to this room were there is a box (to the left) and your see a couple piled ontop of eachother (to the right) then you see a wall (no higher than your hips (from just geussing)stand inbetween the boxes and then aganst the wall then they go all around you and stop about (a yard or two away) (just geussing) some go all the way on the other side of the room! Hope you havent found allready becuse I think it's cool (you can still do it if you get a fire sale) unfortunally I don't have anywaqy to show a vidieo so read carfully and works against dogs too and in both multi and single player.
Basically, choose "Facility", then wait by the window at the opposite side of the room from the stairs. Let a zombie through the window, then headshot it with the knife. Repair the window and repeat. When you have around 7000 points, or less and a Fire Sale, wait until there is only one zombie left in the current round. Throw a knife at him to weaken him, then remove the first barrier (honestly it's a sofa, could just climb over it without paying 5000 points), run up the stairs and go to the mystery box. If the first weapon sucks, keep trying. Assuming you get a useful weapon (any automatic), camp by the box and kill as they come through the door or window to your left. Using this tactic I got to round 12. Good luck and aim for the bloody head.