The Orange Box

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

For this achievement you can ONLY USE THE GRAVITY GUN, you can switch to other weapons, but if you hit the trigger then you can't get the achievement.

There are many saw blades scattered throughout interiors of the buildings, try and keep one with you at all times as they can take out the zombies in one hit. All in all this achievement isn't to difficult, just be aware of where zombies are and try and keep some kind of sharp instrument close at hand to hurl at them, if not use anything and everything you can.

Also when you get towards the end where you wait for the container that the priest sends towards you so you can get across. At this point the leaping zombies will climb up the pipes to get you, simply run around in circles and they will keep chasing you (not being able to hurt you) until the platform gets to where you are, jump on and you only have one more area to get through after that, which mainly consists of regular zombies and some head crabs.

The way I did this is to carry Dog's ball, the deactivated roller mine, to Ravenholm with me. Using the roller mine is a great way to get through this level because it will roll towards zombies and they will attack it instead, distracting them while you find something to kill them with. Also it kills headcrabs when it rolls over them. Doing ravenholm this way is a tone of fun because you can watch loads of zombies swarm and try to kill the roller mine while you watch from a good vantage point. Just don't set off any explosions close to the roller mine or you can wave goodbye to your little friend.

I just did this achievement, and honestly. Its not hard. Theyre are plenty of objects to toss at the enemy. Try to use weapons that attack 2 or 3 zombies at once, Like barrels and saw blades. Also, When Grigori is on screen, He will do alot of shooting for you. If they're are no traps or weapons, just manuver around the zombies. They're fairly slow, so if you can lead them to one side of a room or hallway, you can usually boost your way out of there.

Health isnt an issue either. Allow a poison headcrab to bite you. This will reduce your health to 1, but will climb back to 100 after the antidote is administered, despite what you started with. Poison headcrabs can't kill you, only poison you again. Be careful with this because it takes a few seconds to heal, and even a small headcrab can do 1-2 damage. This is a valuable stratagy, and can be used elsewhere in the game. Cheating really isnt required.

* Testing that, It doesnt seem to work much outside of ravenholme. But I have done it several times inside of the town. So it might be hit or miss..

The only real hardparts are when waiting for grigori to call the elevator over. Only 2 fastzombies will climb up, and running in circles will keep them busy. When you get onto the elevator, you might take some damage, but they're are healthpacks waiting for you.

When you are in the mines, Ignore the headcrabs, and just keep moving. The achivement will unlock when you are out of the final mineshaft.

hi! I found this video! is excellent
It shows exactly where the level starts and when you can fire a weapon
hope it helps
All credits go for "TurmoilTheCoil"
In adition you can use the invencibility code: LB, UP, RB, UP, LB, LB, UP, RB, RB, UP
With this code you can go for the level and it'll be like a piece of cake