You must kill 1 Order Member.
These are the guys in the mining suits (gas masks) you will first run into them at the Power Station when trying to restore power to get into the Prison.
They come in two styles: either weilding a rifle or a pipe.
Rifle: I'd prefer taking them out with any of your long range guns, as when you get in close he can dish out melee butts with the rifle that are pretty fast and damaging.
Pipe: Simply equip your Knife and repeat the A,A,A,X combo. He will end up dodging a hit or two, but this combo will interupt him from being able to land a hit with the pipe.
After killing one the achievement will unlock.