Outlast

This achievement is unmissable and will come with normal story progression.
You will get this right at the end of chapter one.

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There will be a passage in which you will be hunted by the guy with the electric saw again. You will find yourself in a rather big room with doors to the left and the right. I took the right door, sneaked along the hallway and turned left. After that you will find another hallway to your right take it and follow till it ends. Right there you can climb over the wall ( I did not notice that at once and had some trouble at this point). After you hopped over this obsticle the guy with the saw can't reach you and you can just follow the hallway to the gas valve.

If you need additional help take a look at the video below. (Fast forward to minute 20 or so and you should find yourself in the big-ish room described above)

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video credit goes to: Game Front Walkthroughs

Hope this helps you guys ;-)
Any improvements? Let me know in the comments!