After you unlock the slingshot, go to the top of the Daily Bugle. Put on your spidey senses and you'll see 2 little smoke towers.
Go to them and aim yourself toward the park, click X then keep tapping down on the left analogue stick and eventually you'll launch yourself all the way to the other side of the city.
When you near the end of the flight (when you get close to the water) rapidly tap RT and A and you should get the achievement.