Arkham City- Gotham Base Jumping Achievement

Head to the top of the Wonder Tower, either during the game or after you have completed it, and then glide forward off the ledge. Once you’ve gone far enough out, you’ll hit an invisible barrier and Batman will turn around. When he does, circle back towards the barrier again and keep repeating. If you go too low, Batman will shoot out his grapple and you’ll have to retry.