The strategy for undead assault that will * almost always* work

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. It's best to get the doors open during this time. All doors must be opened by round 5, or this strategy will not work. 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. Make your way as quickly as you can to 200, 0. The zombies will all go to the very bottom edge of the map, then just walk to you in a straight line and will all come from the left.
***The following can be skipped if you managed to obtain a Cz and AK before***
Assuming you couldn't get the box enough times before, you'll try to get your weapons now. But using the box is too risky at the moment. Stand at 200 0 and wait for the zombies to come to you. They will each take two hits to kill now, so once you start hitting them, also start backing up to the right. Make sure to advance back to the left any time you think you have some breathing room so you don't get trapped. You want to stay close to your original start position.
What we're waiting for is a fire sale. It may even take a round or two, but sooner or later, you will get one. When you do, spam the hell out of it until your have your CZ/AK combo, if you didn't get it before.
Finally, the fun can begin. The rest of the game now just amounts to you waiting for the swarm at 200 0, then mowing it down. The rules are simple. If no power ups are active, use the AK, because it does by far the most damage. When instakill is active, switch to the CZ. Firstly, this helps you save ammo in both weapons, but also, the CZ is a very quiet gun so you can easily hear what's going on around you. While you're killing zombies, feel free to move to the left every now and again to get closer to the swarm and finish the rounds faster. At this point, you're almost unstoppable anyway so you shouldn't have problems if you take it slow enough (Running out of ammo also shouldn't be an issue because max ammo drops are plentiful). Do not overdo it though, because you don't want to be caught in the main swarm at any time. You most likely will die even with instakill on. When the current round ends, start running to the right again and return to 200 0. This is because you do not want to be in the spawning area once the next round begins.
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 200 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.