After you get a bomb from Croco, you can reach the back of the Moleville Coal Mine, where you encounter Punchinello.
Throughout the battle, Punchinello will sometimes put Microbombs in front of him. These bombs will sometimes dash forward and damage a character, you can block if you press the button just before the bomb collides with the character. You can attack the Microbombs to get rid of them.
Punchinello has a normal attack where he goes up to a character, pulls his arms back, then punches forward. You can block this by pressing the button while his arms are pulled back, just before he punches.
After a while, Punchinello will destroy any Microbombs still in the area, then replace them with normal-sized Bob-ombs, which can also dash forward and damage a character, doing more damage than the Microbombs. Even though you can defeat these bombs quickly with Mallow's Thunderbolt, more will appear immediately. You might want to just attack one or two of these bombs so there are fewer that can attack your party.
After you deal more damage to Punchinello, he will replace the Bob-ombs with larger Mezzo Bombs. Again, these can dash forward, doing more damage than the Bob-ombs.
Punchinello can also use Sand Storm, which inflicts fear on party members (but if a character equipped the Fearless Pin, they will be immune).
Eventually, after you have dealt enough damage, there will be a cutscene. Afterward, you will get the orange Star Piece.
Go northeast and follow the path, and there will be a cutscene. You can read the note on the wall to learn how to control the mine cart. Then the Mine Cart Ride will begin.