To enter Slow-Mo, press
while aiming your gun. If you have successfully pulled a headshot on an enemy, the slow-mo bar will partially fill, giving you more time in slow-mo.
I chose to do this achievement at the police station in the French Ward. Here, the enemies are standing close together. The location of the police station is as follows:
Enter the police station by climbing over the southern gate. Run a little closer towards the main building before shooting. Your four targets are the two cops standing next to the main building doorway and the two cops standing next to the police car. I recommend using a
pistol for greater accuracy.
Once you have made four headshots, allow the police to kill you. You should respawn at Father James' house. If your car is parked outside, drive down the adjacent road back to the police station.
Rinse and repeat until you have done this 10 times.
Video: