This is a secret area that is only accessible AFTER restoring the wind in the Forest; Nibel. It will also require the climb ability which should be unlocked by this point.
After restoring the wind, go back to the Misty Woods (in the bottom left corner of the map), which can be accessed using Sorrow's Pass. Once inside the Misty Woods, travel all the way to the left of the area. Use your new climb ability to progress up a tree, and then you'll use your feather + wind in order to reach a tree branch. Continue to the left until the achievement unlocks when you reach the ability cell.