You can get this by playing starting with the Ravenholm chapter. You can only use the gravity gun, but like the name suggests, you can use one of the many saw blades lying around to chop up the zombies as they attack you. You just can't use another weapon besides the gravity gun. (Other objects that you pick up and shoot with the GG are fine, too... so is using fire.)
One thing to note... the achievement doesn't unlock until after you've left the graveyard, gone down the shaft, through the water, up the mine, and gone into the light. In other words, you need to be at the next chapter (which would seem to be obvious), which isn't until a bit after you leave Ravenholm proper.
I only say that because I freaked out a bit until I finally got it, wondering if I'd just wasted the last hour. :-p