This achievement requires that you find and talk to Mr. Mushroom in several locations, then complete the game.
Mr. Mushroom will appear at his first location after the three dreamers are destroyed. You must have the Spore Shroom charm equipped when you talk to him so that he will advance to his next location. This charm can be found in a room in Fungal Wastes directly below the Queen's Station Stag Station and requires the Mantis Claws to reach:
His first location is also in Fungal Wastes not far from the Spore Shroom location:
You will hear him babbling as you get close. Hit him with your nail so that he will wake up and turn around, after which you can interact with him. Advance through his dialogue and he will disappear and move to his next location in Kingdom's Edge:
Talk to him here and he will move to his next location in Deepnest:
Talk to him here and he will move to his next location in Howling Cliffs (inside the Nailmaster's house):
Talk to him here and he will move to his next location in Ancient Basin:
Talk to him here and he will move to his next location in Fog Canyon:
Talk to him here and he will move to his final location in King's Pass:
After this conversation, Mr. Mushroom will fly away and bid you farewell. Complete the game to see a bonus scene after the credits at which point the achievement will unlock.