The first opportunity to get this achievement is during the quest, "Liberate Captain Marshall." You will be sent to the Authority Prison. As you are making your way through the prison near the start of the mission, you should be able to hear some of the Enforcers hovering with their jet-packs in the next room. This is the first time you will run into them.
Before entering the room, save your game. Then switch to your Crossbow and use the Electrified Bolts. As you enter the room a jet-pack Enforcer will descend and hover for a split second. Fire the Electrified Bolt into him and watch him fly around. Shortly after the achievement will pop.
If you miss, just reload your save and keep at it until you hit him.
EDIT: Thanks to Hirsute Dave who mentioned that tossing an EMP grenade underneath the jet-pack Enforcers will also take them out.