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 bellow 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 and press the to throw the Batarang and cut the rope to drop the enemy in front of the others.
To elaborate further on "zeroway21"'s comment: After the Inverted Takedown, let him hang long enough to attract the attention of the other guards whilst swinging to an opposite Gargoyle to get away from the scene and to have a clear firing lane for your Batarang. Wait for a guard to get close, Lock on your Batarang target reticle to the Rope, and Throw your Batarang. The achievement should pop up immediately.
You shouldn't have any trouble getting this one. Just follow Comment #3's advise and you will get this one. I got it when I was in the masion, right after I ran into the thugs with the collars for the first time.