F.E.A.R. Files

Simple, like it says, just beat the campaign. The difficulty doesn't matter at all. The achievement just pop once you see the credits.

This campaign focuses more on the horror than the Perseus Mandate campaign so you should have an easy time here.

Here is a great walkthrough for the whole Extraction Point-Campaign if you get stuck:
http://www.gamefaqs.com/computer/doswin/file/932918/47017