I searched and did not find this anywhere so I thought I would share a strategy that I use for solo. I am sorry that I do not have a cap card but I will try to explain it thoroughly and give graphic explanations through ms paint!

Beginning Tips:
First off you will be opening the Thompson side first. I would suggest grabbing a Thompson. You can use the random box, get perks, or link teleporters up first. Whatever fits your style of game play the best is recommended. Note DO NOT clear the file cabinet by Juggernaut. This strategy is good for both rounds survived and score.

For the strategy itself:
You will start on the ledge near the Bouncing Betties wall spot, up the stairs from them on the ledge. (I suggest planting your Bouncing Betties, if you have them, down the Juggernaut/ Bouncing Betty hallway). You must now wait for the zombies to cross the bridge or for both of your betties to blow up and for a zombie to approach you from the Juggernaut stairs. Once a zombie is within a foot or so of swinging range jump down and run to the doorway (right next to the Bowie Knife) and unload on the zombies that are coming towards you (they should be coming in a fairly straight line so you can just unload). Once the zombies are near to you run up the stairs (the stairs for the mystery box spawn) and go to the window that is in-between the box spawn and the stairs(DO NOT STOP AT THE TOP OF THE STAIRS AND UNLOAD ON ZOMBIES). Once the zombies approach you jump down and carefully dodge the zombies that are still running towards you (from the mainframe direction) by simply making a sharp right swoop and dodging and getting ahead of the zombies. Then take the right hallway and then turn towards the Speed Cola machine and run to the small room by the Double Barrel wall gun and stand outside of it for another long line of zombies coming from he left hallway towards you. Once the zombies are close run up the stairs and to the drop off near the Double Tap soda machine and when zombies are close drop off and repeat the cycle till the round is completed.

Upgraded Guns I used: (They all worked fine)
Thompson
Stg
Mp40
Magnum
Ray gun
Browning (Need a lighter gun if using this don't use two heavy guns. I had the ray gun with it so I could dodge and sprint longer.)

Pros and Cons:
Pros: You get a ton of points if you use a Machine gun, Sub-machine gun or Stg.
You don't have to use the teleporter or electric barriers (although you can if you wish)
Cons: You must be alert and on top of your game.
You must be careful on the stairs when approaching the mystery box stairs and prepared to kill a zombie if it pops out of the window and blocks you.
You must be careful when dodging in the main courtyard where the mystery box original spawns (especially when you drop down and are running past the gate).

My Path
[ Register or Signin to view external links. ]

Results:
I have made it to round 27+ but I made a crucial mistake and stopped on the mystery box stairs and I had over 150k points.
There is a much higher level of potential for this strategy but I current do not have time to test it again.

If you have any questions ask away!