Look, we've all been there before. Maybe you're in a shitty team with people that can't be bothered to coordinate and revive each other, because they're all out for themselves. maybe your trusted team members aren't online and so you have to play public games with people you don't know. Or maybe you just want to solo the hell out of this thing and see how much you can inflate your score. This short guide describes how you do this. It doesn't have to be just you, in fact, if a bunch of people do it at the same time, you all get kills and score, and there's less risk of the game ending if one of you fucks up.
Ok, so you've created your game and you've launched. The first thing you do is go to 0 50 and grab your sword. This gives you less points overall, but is far more reliable as a melee weapon than the knife is. Turn on the power, or let someone else do it if you've got people with you.
Now you must survive till round 5. You can pretty much do this just by walking close to a zombie, then attacking with the sword until its close enough for you to slice it. The doors at 50 0, 50 50, and 79 50 must be opened by round 5. If you have any time at all before round 5, try to hit the box and start working on getting a cz and AK combo. You'll want both weapons for what comes next.
Right then, round 5 has just started. If you already have cz and AK, go get the juggernaught perk. It will let you take more damage before you die, so encase you get swarmed you can stay alive longer. If you don't have the right combo yet, keep working at it until you get it. You probably won't survive the trip to the perk without it, because there are a lot of windows along the way. Once you have the perk, get back to 0 0 as fast as you can. The zombies will all come from either the right or above, so you only have two directions you need to watch for.
Finally, the fun can begin. The rest of the game now just amounts to you waiting for the swarm at 0 0, then mowing it down. Try to use the AK as much as possible. It has more stopping power, which you need since the zombies swarm like crazy in the later rounds. Use the cz only as a backup weapon, and even then, it's recommended to at least have instakill active when doing so since its damage isn't very good. Don't try to move too much because all you do then is open up a third direction for the zombies to come at you from. When the current round ends, get back to 0 0 as fast as you can. Don't go too far to the right or up, because there are windows right there.
This strategy will work pretty much all the time. It can be done solo or in a group, if you're in a group and you die, the others can hold the swarm at 0 0 while you go abuse the almost constantly running fire sales to get your weapons back.
A note about reviving when using this strategy. If a team mate of yours dies and they are in the spawning area somewhere, going back to get them is a bit risky. It's best to just divert the zombies to you by waiting for them, then killing until the next round starts. If they are near the killing position you have though, it is advantagious to revive them because they can use the crappy starter weapon to help you get some kills while they go use the fire sale to get the AK/CZ combo back.
Ach du Scheiße, das glaubt man mir nie...