F.E.A.R. Files

Completing an Instant Action map on extreme might seem ridiculous for novice shooters at first, but once you get the hang of it, you'll have no trouble doing it.
The most important thing is to ALWAYS use slo-mo when shooting someone. And take cover when you need to refill your slo-mo meter. Also try to take your distance when you're out of cover to shoot someone.
Prioritise your weapons. Obviously a type-7 or an ASP rifle ranks higher than the submachinegun. Pistols are also very useful.
For this achievement you only need to finish one map on extreme. The easiest ones in my opinion were Clinic and Sprint. Also a quick run through the map on Low difficulty will make sure you won't get surprised.

An example of how it should be done (credit goes to AcidGlow for making the video). This is on Clinic: