Go south to the sea to meet the Liriope and handover the Game Machine. You can now swim the sea. Beat all the Denjuu in the sea for exp and while doing that, talk to the one Denjuu that doesnt fight you. He'll go back to his house on land - just return and enter the house near the treasure chest.
Step into the star circle and then go into the cave to unlock the Antenna Tree door.
Back to the sea and head into the tree. Kai drops by and challenges you, be ready as his 1 partner is L20(all that swimming should have paid off).
