High Above The Clouds achievement in TUNIC

After collecting the Magic Orb from the Frog's Domain and completing the puzzle to open the central gate in the Ruined Atol, you can access the Great Library.

Make your way through this area to the room where you can power up a warp gate, then climb the ladder to access the top of the Library, where you will fight The Librarian. Defeat The Librarian to collect the green key.

This isn't too hard of a fight as long as you've been exploring and leveling up. Just watch The Librarian and dodge when he charges. Most of his attacks are easy to avoid once you can anticipate them.