Yoku's Island Express

Right in the beginning of the game, you'll meet Fosfor in a pond, and you'll be required to feed him a mushroom to progress in the story. Fosfor's lair is down in that pond. You'll need to wait until much later in the story when you've acquired the Dive Fish and Sootling On A Leash abilities. Once you have them both, return to this pond (just east of the beach you start at on the western side of the map) and dive down. Swing across the flowers with the Sootling to find Fosfor. Talk to him to unlock this achievement.