I played the game using a controller so the buttons will be for that, for the PC controls please refer to the manual.You will firstly need to have purchased the 'Inverted Takedown' move from the Waynetech upgrade menu (accessed via the button)Throughout the story there will be many areas with Gargoyles in them, once in one of these rooms grapple onto one of the Gargoyles that hangs of a walkway/reachable area. Now wait for one of Jokers henchman to walk beneath the Gargoyle, while aiming down press
to perform the move.
Once the henchman is hanging grapple to the next
nearest Gargoyle and select the Batarang (
on the d-pad).
Now wait for more henchman to come over and inspect what has happened, when one or more henchman are directly below the hanging one hold
near the rope and Batman will automatically target the rope and press
to throw a Batrang, the henchman will fall and Achievement unlocked.
Here's a video to demonstrate