Easily done in Prologue: The Cleaner
or Mission 1: The Battle of Endor
. While playing as Iden, you'll charge your droid by killing enemies and completing objectives, as shown by the meter which fills near the bottom of the screen. When the meter is full, you can use Y to activate the droid onto an enemy, and it will go and shock them.
The goal of the achievement is to have the droid shock and kill 3 enemies in 1 attack which is easily done with any group of 3+ enemies standing next to each other. You will have many opportunities during the Prologue as your escaping the ship, or it can also be done extremely easily at the very beginning of Mission 1, after using stealth to take down the first 2 enemies. There should be a group of 3 enemies directly ahead of you which will group up often near the door.