I can consistently get to 22+ by myself on the theater map. Its quite easy to be honest. I stay in the begining room for 3 rounds and get all the kills with a knife to get some points. Then I unlock all the doors along the route of the MP40, grabbing it along the way, and I usually have just enough to get into the room and turn on the power. After a few kills I can afford Quick Revive (which is amazing in solo, which, in my opinion, is the easiest way to get up to higher rounds. I will explain why I think this later). I save up for Juggernaut and then toy box until I get some good guns, and usually have a pack-a-punched gun with a good side arm by wave 16-17. I don't get Sleight of Hand until I'm on my third quick revive (or have none left) because my strategy relies more on running around and I am hardly ever pressed into a corner reloading. After I get my third Quick Revive I will usually take Sleight of Hand, though. Double Tap is cool, but hardly essential.
My "strategy" is pretty much running around from the theatre to the starting room, down to the room with the mp40, and then back into the theatre, rinse and repeat; using the pack-a-punch room to rain grenades whenever there is a massive swarm. The electric fences obviously play a key part too, and in the later waves, they usually get most of the kills in a round to conserve ammo. In my opinion, the upgraded thundergun and Ray Gun is the best combo, one good for picking off individuals and one destroying crowds. This "strategy" really relies on running around, and its a lot easier by yourself, as 2 people increases the amount of zombies by quite a bit. Its also harder to run around as the whole draw of this is that the majority of the zombies will be behind you, which will not be true with someone else unless you both run in pretty much the same direction at the same time.
I rarely die to swarms, its often those 2 zombies that are there when I round a corner :|.
My current record is half way through wave 27; I haven't really been playing solo zombies for that long (about 3 days maybe?).
@folg: I like kino better. It's a lot easier to run around there in my opinion, and a lot less corners....
Hmm I sort of do the opposite, I knife the first 3 rounds, run into the alley on the end of the 3rd or 4th round, buy the ak74u as my first gun, finish the 4th wave, kill all but one zombie in the 5th wave, then run into the theater. Then use the box until I get a decent weapon, chill in the teleporter until I get overrun, warp, and then rain grenades down from the pack-a-punch room. Once i'm out, I run to the alley to take on the rest of the wave until about wave 16-17, after that I run upstairs to handle them, I hold out there in the room with the pm63 and that overlooks the theater, and never open the three doors leading from that room through the dressing room to the theater, which yeah means I never get sleight of hand, but it's necessary. I then activate the trap in that room, letting it kill all the zombies coming up, while I handle the zombies entering from the ceiling and back, rinse and repeat.
I no longer use the lmgs, ray gun, thundergun, etc. They are way to boring to use, and just take the fun out.
Also on the round that I went to 28 by myself I only used one quick revive, but I died because I activated a trap from the outside right when I got downed, so when I got back up I no longer had juggs to get me through my own trap and was killed lol.
also I much prefer kino der toten to five.