ViZaRd1676,614 Locked 21 Jun 2010 22 Oct 2011
The easiest way I found to finish this game was to gather all the weapons you can in the first wave, but don't use them.
Instead of that you can mess around meleeing everithing you see in the first wave, and when it is over, run to the stairs al the back and jump to a little platform that is in there.
Then just wait (you can let your controller in the floor or whatever), since the fire zombies will explode and kill the other ones that gather around them, making the job instead of you.
You can also shoot the fire zombies in the head so they will explode earlier if you see it better.
The video shows you the place.
Have fun :)
*The video is not made by me though, so the credit goes to xiNRGix*