At the very start of Act 1 Chapter 3 is your first opportunity to use this ability and a good spot to get this achievement, you will be fighting off Juvies with a pistol and unable to move. Kill the first four Juvies and then wait for the next three to get close to you, once they are all next to each other hold down
. If they are close enough Jack will stun all three and get you the achievement, if you mess up you can pause the game and reload the current checkpoint. You can also use Jack's pulse ability by pressing
to see the Juvies easier, if you are playing on a harder difficulty this spot may not work as they might kill you before they group together.
Here is a video of me doing it if you need help, the first Juvie dies right at the start which is why it looks like I only killed 3.