The easiest way to knock down Zsasz is with the 'remote control batarang upgrade'. Just stand out of his line of vision and using detective mode you'll be able to see when Zsasz pops his head out from behind Dr. Young. Target your batarang before hand just to the left of him with the LT, when Zsasz pops his head out on the left (Batman's left) fire the batarang (RT) and steer it (LS) toward Zsazs's head.
You should be given an upgrade option just as you enter the room, it did for me on two separate games. Also, if you take too long Joker convinces Zsasz to kill Dr. young. If Zsazs sees you it's an automatic fail.
Alternatively you can crouch and take a position at the edge of the doorway and attempt to batarang Zsazs in the head when he exposes it.