This is my best strategy for Nazi Zombies "Der Riese" that I have come up with so far:
Get as much money as you can in the courtyard where you start by shooting zombies with your pistol a few times then knifing them. By level three or four, you should have 2500-3500 points. You and your teammates buy your way to the main courtyard where the random weapon bin is, picking up a trench gun off the wall as you go. Turn on the power, get random weapons, etc. That main courtyard (with the random weapon bin) is an excellent place because all the zombies come from one direction. Stay there until you are rich enough/have good weapons/the random bin is gone, usually level 6-8.
From there, you all go together to link each of the teleporters to the mainframe, picking up perks as you go.* Buying Speed Cola and Juggernog is an absolute must, especially Juggernog (buy Double Tap Rootbeer if you have enough cash). After all the teleporters are linked, your entire team should meet back in the courtyard where you started. Your stronghold will be by the mainframe, right in front of the Pack-a-Punch machine. Stay there for the rest of the game, upgrading your guns whenever you have the money. Because all the zombies come from pretty much one direction, you all can stay backed up against the Pack-a-Punch machine for a pretty long time.
*OTHER TIPS: to make the "link teleporters while grabbing perks" step easier, try keeping around the last zombie of the round. Late in the round, grenade a group of zombies and kill all of them except one that's crawling. Make sure none of your teammates kill him! As long as you keep him alive, the round will not end/the next one won't begin, and you will have a virtually zombie-free map to move through.
That's the best I've seen so far. Thoughts? What strategies do you guys use? Post 'em.