To earn this trophy you must use Batman's ground takedown to knock out Zsasz. If Zsasz sees you just once, he'll kill the hostage and you'll have to restart from the last checkpoint. You'll need to get behind him by using the vantage points to position Batman behind Zsasz. Once in position, you'll get the option to Batman's glide attack on him. Doing so will knock Zsasz down temporarily. Get close to him and press
to use the ground takedown. Once Zsasz has been knocked out seconds later you'll unlock the trophy.