You're gonna have to climb the clock tower above the Arkham Mansion to get this one. Facing the main door to the interior of the Mansion, grapnel to the platform above (the one with a metal grate embedded above it) and then grapnel again towards the rooftops. Once there, head towards the clock tower (no way to miss it, you can see it in the distance). Once under the tower, look up and grapnel, then grapnel again to reach the very top. Once there, glide down towards the east grounds. You will get the achievement as soon as you hit the ground.