F.E.A.R. 3

In the very first mission: Prison you will encounter a door and you will be told that you can kick down doors if you press the melee button. You don't need to kick the door down though (it is up to you).

When you open/kick the door you will see 3 enemies to the left sitting at a door playing poker. As they are getting up it will enter into slo-mo for the first time. Just aim carefully for the head on each enemy. You should have more than enough time. If you messed up and didn't get the achievement you can press "Start" and load the last checkpoint. It will only be seconds before the spot.

Also this can be obtained by either character even if Fettel doesn't have a possessed body because he can get headshots with his blast. If you are playing Co-op make sure you friend doesn't posses or kill anyone.

Note: The achievement only unlocks for the person going for it.

I have made a video on how to easily get this achievement.

In the Poker Room of Prison, kill all 3 enemies with headshots before slo-mo ends

Character - Point Man
(you can get this as fettle as well if you are playing coop)

Step 1 - Start up the first mission on the easiest difficulty
Step 2 - Learn how to melee (L3 or Left Analog Stick - You will learn this on the first person you kill because you have to melee him in the back.)
Step 3 - Once you melee him in the back you will have to fight a guard by a lever kill him and then activate the lever. (Alma should pop out and possibly scare you)
Step 4 - Once you hit that lever immediately look down the hall way you were heading, because 2 guys will come charging through the door. Kill them
Step 5 - Once you kill those 2 head up the stares and once you open the door throw a grenade (RB) at the 2 guys standing to the right of you and take cover.
Step 6 - Once you kill the 2 with the grenade another guy should burst through the door so take him out as well.
Step 7 - Now before you move on I recommend you grab the SMG or you might have trouble getting 3 head shots with the pistol.
Step 8 - Head up a bit until you reach this door (it will say press X) DO NOT PRESS X LIKE I DID YOU WILL HAVE TO MELEE THE DOOR THIS WILL MAKE YOU KICK IT OPEN AND AUTOMATICALLY PUT YOU INTO SLOW MO
Step 9 - Once you are in slow mo (Make sure to have it full before you burst through) look at the 3 guys to your left playing poker and shoot each of them in the head (final shot that kills them must be in the head) and ACHIEVEMENT UNLOCKED

NOTE: It may be hard for some people to get 3 killing head shots on your first time so if you don't get the achievement press start and reload checkpoint. MUST KILL THEM WITH A HEAD SHOT CANNOT JUST SHOOT THEM IN THE HEAD AND THEN KILL THEM BY SHOOTING THEM IN THE CHEST OR SOMETHING AND OTHER WAY AROUND TOO IF YOU SHOOT HIM SOMEWHERE ELSE FIRST IT'S OKAY AS LONG AS THE LAST SHOT IS A HEAD SHOT

You should have no trouble with this achievement as it is very simple and it shouldn't take you more than 5 tries. Remember to kick open the door and shoot the 3 to the left of you. The reason I made the steps so long is because I tried to include everything possible that way nobody will need help understanding the achievement.