After you defeat Johnny and win the Star Piece, you return to Seaside Village, where you must battle Yaridovich.
Yaridovich can attack with Water Blast, which hits everyone for high damage. Flame Stone and Willy Wisp each hit a single character for high damage. Static E hits all characters for low damage. Meteor Blast hits all characters for moderate damage. Bolt hits a single character. Yaridovich also has a regular attack where he uses his head as a spearhead. You can block it if you press the button right before Yaridovich pushes the spear through you. He can throw a small white orb at you, which you can block if you press the button just before the orb hits you. He can attack with Pierce, which you can block when he has started going back to where he was standing before the attack.
Mirage Attack makes Yaridovich split in two. One of them is fake. The real Yaridovich has higher defense, so if it seems like you're doing a lot of damage, switch to the other one. Eventually, the two Yaridoviches will merge back into a single one.
After you defeat Yaridovich, go east to find the Shed Key, and a note on the trees.
Back in Seaside Town, go to the southwest house and check on the door to unlock it with your Shed Key. There will be a cutscene. The real Elder will give you a Flower Box, so be sure to use it.
Now the shops in Seaside Town are open. Beetles R Us gives you rewards based on the beetles that you can catch at the Booster Hill obstacle course. The Weapon Shop and Armor Shop have some stronger items. The Health Food Store sells healing items. At Mushroom Boy's shop, he can eat your mushrooms (such as the mushrooms you can buy from shops, like the Health Food Shop next door), and if you have any special mushrooms, he will give you Rock Candy or Maple Syrup or Flower Tabs. Most mushrooms aren't special, however. There is an Accessory Shop next door to Mushroom Boy's shop.
Go to the northernmost house and talk to the Elder to learn about your next destination. Leave town and go north to reach Land's End.