String up one henchman and drop him to surprise a second (any play mode)
You can only earn this once you buy the skill upgrade ability to perform inverted takedowns (black title with white eyes). Either enter a challenge or story mode, jump up to a gargoyle, and hit to hang upside down. Once an enemy passes below you, press to drop down and leave them hanging from the gargoyle.
Jump to another gargoyle by hitting the , then select and aim your Batarang using the . Aim for the rope holding the enemy; then press the to throw the Batarang and cut the rope to drop the enemy in front of the others.