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 (in the leftmost dead end) that doesn't fight you. He'll go back to his house on land - just return and enter the house near the treasure chest.
Talk to him and you will be warped to an otherwise inaccessible area of Bamu Sea. Go into the cave and step on the switch to unlock the Antenna Tree door, then step into the star circle to return to the house.
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).
