Well I am pretty sure I still haven't done a brutal kill since the intro to it and I'm on chapter 5. 
 
Can someone explain what the vague instructions were actually telling me to do in the beginning?

Shoot a light shot at a generic baddy and run up to them and press X.  There are 5 different animations for this, all of them net you an achievement.  Another is blowing off one of their legs then running up to them while they're on the floor and stomp on them with X.

@mooseifer Thanks!