On Der Riese you pass the Thomson room and go up stairs to the next room. Then you go to the corner on your right with some desks.

You will look at the desk that is slanted and there should be a radio at the top. If you press X at anytime when you are facing at looking at the radio all the zombies will come through that one window in the room.

You can not leave the room while the zombies are coming or it won't work. I suggest having Bouncing Betties because if you run out of ammo you have a back up plan.

I strongly suggest having a gun with a lot of ammo and leave a crawler after every round so you can get more ammo and maybe hit the box.