If you play solo alot, then there is a path that you can take around the whole Der Riese map and you will get hit like 2 or 3 times, but you won't ever die from it.

You start at the catwalk and run through the open window down past the Zapper and go right. Never go left!.

Then either run down to the trench gun side of the Speed Cola side, depending on which way the zombies are coming from you will be able to side step alot of them as long as you dont stop running or run into a wall.

Just run back up to the bridge, wait a moment, then drop and do it all over again.

I got to 46 solo with only 2 guns.