Yeah, I was talking about the fly trap
I don't know where it is exactly or how you activate it but my friend opened the doors on the left, then we got to level 15 or so and went back through there to upgrade a weapon, he sees a zombie at a window starts shooting it and then we suddenly get the trophy and he sees the Fly trap fly and go somewhere.
Here is what I do on Der Riese:
1st level: Shoot zombies 6 times with the Colt and then knife. let at least 1 zombie get past the window and you should get something like max ammo etc.
2nd level: Same as above but shoot 8 times
3rd level: Depending on how much money you have, you can open doors and get a weapon or repeat what you did on 2nd level
4th level and above: Open doors to get to the mystery box, get a random weapon, turn on the electricity and stay there killing zombies until you have enough money to link the three teleporters, link them and go to where the mystery box is continue going forward till you reach the room where one of the teleporters is, go up the stairs and kill the zombies from there, if you have money and want to buy something leave one alive and go buy what you need, then go back and kill it