This achievement is pretty easy to understand: Play ONLY Gravity Gun through Ravenholm.
It's not very hard, there's a lot of good items to pick & throw to ennemis (especially a lot of "Saw Blades" you can find on the floor, with them you can very easily OS (almost) all ennemis you'll meet on Ravenholm).
Harder part of this achievement is at the end of Ravenholm, when Gregory is sending you the "Cable Car" to join him. You have to wait for it and many "fast zombies" are assaulting you on the roof and the hatch you used to come up can't be opened from the roof.
The best way is to jump down from the roof and stay in Ravenholm streets while killing fast zombies. Then, go back up on the roof and enter the "Cable Car" to meet Gregory.
If you did it right and if I well remember, achievement will pop when you're in the "Cable Car"!