The first time you can get this is during Level 8: The Inner Keep. You'll meet your first ever Elite enemy here, and it will be pointed out to you, stating they look tougher to take down. Unlock the vent in front of him, and you'll come out the other side of the door he's guarding, right by a switch.
Throw a dart at the light out here, and wait for him to come and investigate. Once he reaches the wires hanging from the ceiling, flip the switch to electrocute him to stun him. Flip it again to turn it off and finish him off to get the achievement.