Paper Mario/Chapter 8: A Star-Powered Showdown

Star Way
After talking with the final Star Spirit, start the long journey back down Shiver Mountain, all the way to Shiver City. As you pass the save block just outside Crystal Palace, you can step of the side of the cliff to find a star piece.

Go to sleep in the Toad House in Shiver City, and when you wake up there will be a bunch of items on the floor. Go back through the pipes and into Toad Town (using the shortcut), and north to the very top of Shooting Star Summit. Don't bother buying any items in Toad Town, since there is a much better shop in Star Haven.

Here, you will be able to take Star Way all the way to Star Haven. Use Sushi's water attacks to defeat the Embers along the way. You may also want to re-equip the Ice Power and Fire Shield bages.

Star Haven
On the lower level of Star Haven, there is the star's version of Toad House, and at the top of the castle is a shop with several high-powered items. You'll want to make sure you're fully healed and stocked up, since we're heading to Bowser's Castle in a moment.

Follow the path round to the right, and go into the Star's castle. Here, you will be given the Star Spirit's final power, and a ship that will let you fly to Bowser's Castle.

The Dungeon
Head through the door to the front of the castle. Unsuprisingly, the front door is locked, so instead go through the small door to the right. Defeat the Koopatrol to get the key. Watt's Electro Dash is especially helpful against their strong defence.

Unlock the front door, and precede through the castle, defeating the various Koopatrol and Hammer Bro you encounter.

Eventually you will come across the Guard Door, who will trick you into falling through a trap door into the dungeon. Use Bobmette to blow out the wall and escape.

The Lava River
In the second room, you'll need to use Kooper to activate the switch for the moving platform, and Parakarry to fly across the large gap by the middle door. For the second switch, you'll need to use Lakilester's ability to fly to the platform and hit the switch, then fly back round to the left and up the stairs to jump on the new moving platform. Use Parakarry's ability to get across the next string of platforms.

In the third lava room, you'll once again have to use Lakilester's ability to travel across the lava (which feels awful - it's clearly painful for the cloud!). You can use Kooper's ability to grab the Mystery item, but will need to ignore the Thunder Rage for now. On the next moving platform, use Bow's ability to hide from the waterfall, and the Parakarry to get across the gap. At the end of the room is a locked door, but we can't get through it yet. Instead, use Lakilester to fly through the door in the lava.

At the top of the stairs in this new room, you'll find a Magikoopa and gang of Koopatrol protecting an important switch. Defeat the gang (strong all-enemy attacks, like Star Storm and Air Raid, are helpful here), and press the button. This will block the lava fountain, causing the lava to cool and harden. Mario will now be able to walk across it.

We can now reach the locked door, but we're going to need the key. Double back on yourself, all the way to the second lava room (jumping up to grab the Thunder Rage on the way past). Remember the door we flew past earlier? Mario can now walk to it, where you will find a chest containing the Castle Key. Go back to the locked door and use the key to get through it.