Mafia II: Definitive Edition

I got this in chapter 12 sea gift.
Real easy; After you get in the shoot out with the cops. You'll follow Henry and Joe up a flight of stairs,across a cat walk and into a building. Run down this corridor and duck under the window in the corner. They'll be exactly 5 guys shooting at you from the ground go ahead and take them out with headshots. What makes this area so good for this achievement is no one will be taking your kills.
I hope this helps.

This achievement can be done anytime there are a few places this can be done however the earliest and possibly easiest for this ach is right on the first mission when Vito is in the army. After about 5 mins you will be on a balcony with vans driving past with 4 enemies running once they all take cover aim for the head and shoot this can take a few tries but once all 4 are dead an enemy will appear on the balcony shoot him the your achievement will pop. I’m gonna add a video if you guys will find it easier.💎🏆

Load up chapter 1, normal difficulty, skip the credits and the first cutscene and simply headshot the first five enemies of the game quickly with your rifle.
I suggest starting with the leftmost soldier first as he goes in hiding otherwise, then the closest to you before he gets strangled, then the three soldiers by the steps.
Reload if you fail, the checkpoint is right where you spawn. This is way quicker than any other solution I’ve seen here.

I was able to unlock this achievement near the end of Chapter 10 Room Service. Once you have taken the elevator back downstairs, you're going to work your way back towards the hotel's laundry room. I was able to get 5 headshots in the last area right before you enter the car garage and find Marty killed. There will be about 6 guys in this area so it shouldn't be too difficult to do. One guy will be on the cat walk to the right but the rest will be ground level, as you can see in the video. The achievement did pop after I killed the last guy but for some reason my Xbox doesn't record it.

THE EASIEST WAY TO COMPLETE THIS ACHIEVEMENT:

I know I’m years late to this thing, but there’s actually a mission designed just for this challenge in Joe’s Adventures.

If you have the DLC, go back and replay (or play) the “Supermarket” mission.

Play through until you enter the supermarket offices upstairs. Upon exiting the cutscene, you will be crouching behind the short wall overlooking the supermarket, and there will be a special gun (Karabiner 98k) leaning up against a box for you to use. It’s much like the M1 Garand, but with a touch more zoom and perhaps power (not that power matters when you’re nailing headshots).

Grab it and do your best to kill all of the targets in the room with headshots. You should be able to get 5 fairly quickly. You have good cover and an elevated position. Some cops take “cover” in exposed positions, so it’s not too hard to get 5 easy, quick headshots.