When you start off head to the wall with the falgs on it. Go to the right hand door and destroy the bottom two pieces off glass.

Once you've done this repair the window. Then jump up to the first board, then the second, and fially the doorframe.

Once you're up turn around and jump to the table, but jump to the next one so the zombies can't get you.

Here's the interesting part, the zombies obviously can't get you while you're in the centre of the table, but what you may not know is that after a few minutes the zombies will start to die.
Therefore you can achieve a very high level without even firing off a shot.

I got to level 30 before I got sick off it, all without firing a shot, enjoy.