Now that the papercraft battle is over, head east to Doop Doop Dunes. Starlow spies Bowser's Castle in the distance. Inside, a Hammer Bro asks a Paper Hammer Bro how a paper hammer does damage. The Paper Hammer Bro offers to try it out on him. The other minions are meeting their paper counterparts. The Magikoopa and Paper Magikoopa assemble the minions into a truly massive army. They give their battle cries while Bowser Jr. complains of boredom. Paper Bowser Jr. shares his sentiments. He says they'll get a reward for watching the princesses, but it's boring. Bowser Jr. asks where his paper counterpart is from. He says he and his dad flew out of the same book. Bowser Jr. wonders if Paper Bowser Jr. would have to go home if they found the book. He thinks that is likely. Bowser Jr. wants to keep hanging out to stave off the boredom. Paper Bowser Jr. has a solution: destroy the book. Bowser Jr. thinks their dads would like the idea as well, but neither know where the book is.
In the cage nearby, Peach wonders if there's a way out. Paper Peach slips through the bars with ease. Bowser Jr. wants to find the book once they're done watching the princesses. Meanwhile, the Bowsers appear in front of their armies, saying they're unstoppable. Paper Bowser wants to be in front. Bowser's plotting domination of the Mushroom Kingdom.
Back in Doop Doop Dunes, Luigi spies Nabbit. Dash to chase after him. While dashing, the bros can jump simultaneously with (X). Nabbit dodges twice, but drops the Drill Shell Bros. Attack. Starlow suggests chasing Nabbit for more Bros. Attacks. Chase after him into the next area and the Magikoopas will drop cardboard platforms to block your path. They doubt the bros will reach Bowser's Castle. There are Spinies in the desert. Use hammer, Bros, and Trio attacks to defeat them. Mario made it to level 6 and ranked up to Shell Rank. There are bonuses you earn with each increase in rank, but some may only be equipped by one bro, such as an increase in stat. Go to the northwest side of the area to find a gap. Send Paper Mario in a few times for some helpful items. Reach the higher ledges in the area using the cardboard platforms and you'll reach a red switch. Hit it to make a platform rise up so you can reach a higher platform.
Walk around and you'll find another switch. When you try to hit it, Nabbit will come out of hiding and steal all of the bros' hammers. Chase him down by dashing, but he'll toss spike balls you'll have to avoid. Slide into him to get back a hammer. Do this twice more to get all three hammers back. You'll also get a new Bros. Attack: Rocket Blast. Nabbit runs off to the south. Hit the switch so you can follow him. Bowser's Castle is due east of here. Near the slight hill with the many coins, you'll find Nabbit. Chase him up the sand flow and something drops from the sky. It's a Pokey and a Paper Pokey that looks like Tower Power Pokey. They knock the cardboard loose from the sand flow and send the bros flying.
Starlow says they have to go back to where they came from, but hit the giant stone block can't be broken by Mario's hammer. Luigi and Paper Mario join in to no avail. Paper Mario gets an idea. The bros must use the Trio Hammer by pressing (A), (B), (Y), in order. Stand next to the block and execute Trio Hammer to break it. Starlow thinks the move can be used to flip things over. Break the blocks in the area and walk across the bridge to the right. The Pokeys will knock into the bridge, sending the bros plummeting. They land in Doop Doop Dunes Grotto. The Pokeys are standing nearby and won't let the bros pass without a fight. The bros will need a way to jump really high.