The bros have destroyed Bowser's papercraft, but Bowser sent the Koopalings after the bros. Before dealing with them, go west. There are dirt walls and Shy Guys here. Climb the dirt wall and Drill Dash through the maze of saws to reach a gondola. Use Trio Hammer on its purple button to send it flying into the spiked rollers blocking your path back east. Go north. Grab the Bob-omb to the right and use it to destroy the Bowser statue ahead. Pull out the bridge and keep going. There are boost pads here. Grab another Bob-omb and use them to reach another Bowser statue. You can get a Bob-omb from the right side. Pull out another bridge. There's another gondola here. Grab it, then use Trio Hammer. There's another statue to the north of the first one that leads to items. Use the pipe to return east quickly. Hit the ! Block to lower the gate.
Go east. Dash across the gaps using the yellow platforms. The platforms ahead won't be in a straight line. Walk towards the back and you'll find a narrow path with saws. To get over them, Trio Glide. It's okay if you land in the middle. The saws will hit you, but won't send you back to solid ground. The next narrow path allows you to Trio Drill underground. Use the Trio Hammer on the gondola to the left. A blue platform will take you back to where you entered from the west. Go north. You'll have to Trio Glide across some gaps, but there are fans blowing periodically. You'll need to have the fan blowing for the last glide. There's a Mechakoopa and a gondola ahead. Use the Trio Hammer on the gondola to form a complete path for the bros. Return west using the pipe ahead.
There will be enemies in the main hallway. At the end of the long path are Morton, Iggy, and Lemmy. Iggy wants to push something. Lemmy says it's his job. Morton notices the bros and tells someone to do it. Iggy pushes the button. Morton says it will destroy Peach's Castle. They're referring to the massive cannon at the front of the castle.
In Peach's Castle, the princesses are thinking of the bros. Back in Neo Bowser Castle, Iggy remembers he was supposed to set a timer. He thinks five minutes is good. The bros have to smash the cannon before time runs out. Enemies will drop in out of the sky for battle. You can't flee the fights, so attack quickly. Using Paper Mario's hammer works well. Go south and dodge the Fuzzies by walking between them. Other enemies will attack you, but you can avoid the battles. Go as far south as possible and you'll find the cannon. The timer will stop here. Press (A) to start the Trio Hammer, which will dent the cannon so it can't fire. Starlow thinks they shouldn't stand directly above the cannon when it's about to fire. The bros leave and the cannon blows itself out. Walk north and take the elevator up.