I've been looking fro strategies lately and found a great solo one.

this works from round 10 and up (i got 30 on first try!!!)

Strategy #1

1-i start at mainframe and run up to double tap ledge next to bridge,
2- camp there for a second and then drop down and go to the m1a1 hallway, there you can shoot them if you like
3- Turn into the trench gun hallway and go up the stairs and stand on the ledge opposite from start, shoot again if you desire then drop down
4- Run down and into the speed cola area but instead of going to thompson spot go to mainframe and run across back to m1a1 hallway, and into trench hallway
5- instead of going down again go acrss bridge and around the big circle(thompson,mainframe,trenchgun,bridge repeat)

i got to 30 with flamethrower, and ray gun.

Strategy #2 ( my new favorite)

1- start at mainframe and make your way to double tap ledge.
2- drop down ledge once a big wave of guys come at you.
3- run down m1a1 hallway, i like to shoot there.
4- go down trench gun hallway, and up to the "betty Ledge" opposite from the douple tap ledge
5- once the guys get there, drop down and go through the little doorway leading to fg42/speed cola area( you can go other too, it works just as good). !!!sit outside of the doorway, right infront of the speed cola machine until the group of zombies arrive!!! don't go to far ahead or else some zombies will go around and ambush you.
6- go to thompson room and turn on gate there.
7- as gate is destroying them i go up to the double tap ledge again and wait there til another group of zombies arrive.
8- Repeat as many times as necessary!!! (The most times i needed to do a cycle was 2)

Guns- Flamethrower and Ray Gun. Shooting zombies with flamethrower while making your way down m1a1 hallway can easily gain you almost 4000 points.

If you still need better guns you can just shoot zombies with FlameThrower more often.

I got to 28, on first try with this. Died with 41 000 points!!! Reason i died i went A.D.D. and didnt stop at the FG42/Speed Cola area and got swamped so fast.

I prefer the second strategy, it is much easier and less complicated.

I hope you enjoy these strategies. Thanks