In the last part of Dark Carnival when you get to the concert get a pipe bomb and a meelee wepon (guitar recommended), then start the horde.

If you are standing looking foward from the mike to you right is a tower, get on top is it and throw the pipe bomb at your feet then rapidly press the Y button .

Then you will fly back into a window (get to the back room once in as zombies will spawn).

No zombies, special infected or tanks can find you and just wait it out.

Note: this does not always works if you lucky you will get it on the first try but it takes 2-3 and if not keep trying it works.