F.E.A.R. Files

Simple, like it says, just get to this point in the campaign. The difficulty doesn't matter. The achievement just pop after the screen fades out at the end of the mission "Underneath"

This is the first of six "progress" achievements so it should unlock real soon.

Here is a great walkthrough if you get stuck:
http://www.gamefaqs.com/computer/doswin/file/941968/51786