When you go down to the box, look at it ( get a gun if you want to) and after the end of the round/ very beginning of the next round, turn 180 degrees and walk forwards. Then look right to the window and walk to it and all of the zombies will come to that one window. If you want to hit the box in a round I suggest you do that when there is only one zombie left.
Get the ray gun or another OP wep and go ot the balcony near the mystery box room on the left side of the map and from there whenever a round starts turn on the barrier and shoot through to get points so you can keep doing it
Once you get 1500 points open up door with double barrel shotgun then the next door you will see an air vent to your left go to the right of it and pause game the resume you will need to do this repeatedly and then you can go throw. Don't turn power on or open doors