Buy the Olympia (cost - 500pts) then stay in the first room until round 4-5. Then, buy Quick revive.
After you've done that, open the door upstairs (cost - 750pts), and if the box is in there, camp until round 8. If not, go to the room with the pictures and camp in front of the barrier.

When you have approx 3500pts, open the next door (cost - 1000pts), and if the box is there, open it a few times so you get good guns. If not, camp by the door.

When you have 5000pts, open the door behind you (cost - 1250pts) and camp by the door in the far corner. The box may be in there, but whatever you do, dont open it if it is urgent.

Open the next door (cost - 1250pts), then camp by the power switch, but do NOT turn it on unless you are crowded. Link the teleporters, then when you have 6000pts, get in the big teleporter and upgrade a gun.

TIP: If ever you get downed, rebuy Quick revive
NOTE: This guide is for Solo use only! DO NOT use it on co-op zombies!