This achievement is unlocked upon taking down Zsasz in the Patient Pacification Chamber.
This achievement is a part of the story and therefore is unmissable. After defeating every henchman at the beginning of the game, Joker will allow you to go through a hallway, keep going the linear path, and eventually you will reach a room in which Zsasz has a guard hostage. You have to go up the stairs and onto a balcony overlooking Zsasz. Using Detective Mode
, look at the gargoyles located near the ceiling on the sides of the room, and you will be told to grapple up to them. Do so and grapple onto the next one. You can finally glide kick
Zsasz and knock him down and then use a ground takedown
+
to take him down.