F.E.A.R. 3

This one's pretty easy to just obtain over time. You must complete challenges to get this achievement. You can look at your progression, or just generally check your challenges during the campaign by pressing the "back" (<|) button. Look for the ones that give you a great amount of points, such as:
>>Using active cover for a long period of time
>>Pulling off headshots
>>Using your Slo-Mo from full to empty
>>Fighting with melee attacks
>>Stabbing enemies in the back

There are a lot of different challenges you can complete for points. Also, finding these dead Armacham bodies, along with some other dead bodies that will allow you to "PsychicLink" which will also give you points. There is a separate achievement for using these "PsychicLinks" although, I'm not quite sure what they do. I got this achievement somewhere around the second or third level.

Note: Although you can rank up during online multiplayer, you cannot complete challenges online. Challenges can only be completed during co-op or solo campaign.

GOOD LUCK!