The bros have reached Neo Bowser Castle. You'll find Broozers here. They're tough enemies with powerful punches and increase their power every time they take damage. There's a button in the center. Hit it to make part of a panel light up on a fuel cell. Go west and you'll see another button as part of a maze. There are Paper Broozers here. Walk to the south to reach the maze. The sign says that (A), (B), (Y) flips yellow panels and (X) flips blue ones. What that means is that while you're moving to another panel, press the three buttons simultaneously when you're jumping from a yellow panel and (X) when you're jumping from a blue panel. Hit the button, then the ! Blocks to open the way back. Return east. Hit the button there, then walk south.
You'll find Nabbit here. Try talking to him and some minions will report that Nabbit's been stealing Bowser's stuff as well. Chasing after him is optional. Once you catch him, you'll have to battle. He'll toss coins and spike balls, both of which can bounce. He'll throw his bag at you, then pull out paper enemies. He'll also sneak up behind the bros and steal their hammer if they don't counter. After Nabbit takes a hit or two, he'll run away. Chase him again and he'll start a battle with some Broozers. He'll heal the bros each turn. Defeat the Broozers and he'll run off again. This time, he'll run to the west. Catch him to start another battle. He'll run away, as before. He'll start a battle with a Shy Guy, and fight alongside the bros and heal them. The easiest way is to run from the fight when he's battling against you. The last fight is the one you can't flee from. You'll get Peach's Peppy Gloves and the Bros. Attack Mega Thwonk for defeating him, as well as however many coins you earned during the fight.
Go east. There are many moving platforms moving past the button. At certain points, they will drop away. Use (X) to jump platform to platform. Hit the button and an Up button illuminates. Jump on the other set of platforms and return west. Hit the up button and the platform will rise to a different level. Walk to the north and Bowser is impressed the bros have made it this far. The Bowsers and Kameks appear. Paper Bowser says something has happened. A shadow falls over the bros. Up in the sky, the papercrafts are wrapped in duct tape. Toadette's been captured as well. She orders the Bowsers to release her masterpieces. Paper Kamek brings in the secret weapon: the enormous Papercraft Bowser. It breathes fire on the papercrafts. The Bowsers will be waiting for the bros, should they want to fight.
Toadette is quiet next to the smoldering ashes of her work. Luigi tries to talk to her, but she silences him. She has a great idea, but she'll need the bros' help. She needs them to collect the Fire Ore found around the castle. Fire Ore are the flame-like stones burning in torches around the castle. The bros can safely handle them. Toadette says Fire Ore is fascinating. Harnessing the ore's power will allow her to build a special papercraft. She says they'll need three units of the stuff. She saw some on the lower level, so the bros head there first. The Fire Ore is to the southeast. Iggy swipes it and Bowser asks what Mario's thinking. He won't let someone steal the ore he's stolen. Iggy challenges them. Starlow flies off to find them something to fight back, so the bros must toss bombs at the Koopalings and Bowser to keep them at bay. Starlow brings in the bombs. Iggy will toss three bombs at the bros at a time. You'll also throw Starlow at one point. Iggy will admit he's failed, so Wendy steps in. After Wendy takes a few hits, she calls Bowser for help. He throws one large bomb. Once he's defeated, he tells the Koopalings to never speak of this to anyone. Bowser acknowledges the Fire Ore is pretty, but he's not sure how it will help the bros defeat his papercraft. He tells them to leave it alone.
Go to the southwest to find the other piece of Fire Ore. Morton appears. Mario must use his hammer to return the regular cannonballs Morton throws. Use Trio Hammer for the huge ones. Once that's over with, you'll get the last bit of Fire Ore.