Go to Pikachu's Daring Days and select Cobalt Island (The Treasure Hunt, Part 2). Go to the other side of the island and look around. Eventually, you will find a path leading up to a pirate fort. Try to keep Pikachu from finding the treasure chest. Using view mode, tell Pikachu to "Thunderbolt" the cannon facing away from the treasure chest. A loud blast will ring out. When the smoke clears, you will notice that all the chests will be unearthed, allowing you to simply follow the directions on the signs. If you did not open the chest inside the fort, opening it now will give you a pirate's sword.