I made it to round 35 on Der Riese.
TIP: If you have monkey bombs, throw one past the electric barrier while you jump down from the balcony in the Teleporter C room. Then run to the electric barrier and activate it. Then run back to the balcony. It's kind of scary because you have to run through zombies, but they shouldn't be attacking you. So just pass by them or shoot them as you go.
And you have to do this really often. Like... at least 2 or 3 times per round when you reach lever 29 or more.
So to do this properly you need to have at least 1 or 2 guys using monkey bombs. 3 or 4 would still be better though.
And please. To anyone who still plays like an asshole. Buying bouncing betties IS NOT an option. YOU HAVE TO buy bouncing betties so you can plant some in back up spots.
I played with a retarded kid yesterday who was not using them... and he also was eating breakfast during the game... letting us take care of all the zombies while making us loose points because when he died we all lost points.