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5 G

*This can be achieved before the end of the game.

While you're on the way up the Wonder Tower toward the end of the game, go to the large ledge just before you grapnel up to the maintenance hatch and jump. If you can't get it during this part, try going back up after the end of the game.

The key is to hold back on your left thumbstick (or forward, if you're weird and don't invert your y-axis) from the very beginning of the jump. I didn't do this the first two times, and I failed. The problem is that the game won't let you go all the way to the ground. At some point, Batman will shoot his Grapnel and pull himself back up. So jump, pull back, and in the moments where the game makes you fly up, circle back to the left until you're facing the city again.

One more thing... get it on the way up, especially if you know where the collectibles in the tower are. Otherwise, since the game won't let you glide to ground, once you've climbed up just to get this one cheeve you have to climb back down manually, down the elevator, through the subway, etc. Kinda annoying. Save yourself the hassle and only climb the tower once. (I ended up climbing it 3 times. Grrr.)

Here's a video that I didn't make but shows how it works: