First, purchase the stun effect for your staff so that stuns will radiate out to nearby enemies. Then when 3 stunable enemies are near each other, target the middle with you stun blast from you staff. Should the blast hit the other two, you will get the achievement and 3 helpless foes. Also make sure you have plenty of stun ammo, as it may take you a few tries to get it right.
While they are out cold, you may want to see if they line up for the Kebabed in Enslaved: Odyssey to the West