The Underground Ghost Ship

First, you need to use the cannon to launch yourself to the planet where the Octoguys are. Try to aim through the Rainbow Star if you can – it'll let you run right through all those Octoguys without a problem. Grab the Fire Flower, and then quickly use the Sling Star to warp back to the starting planet. Remember, the Rainbow Star ability is on a timer, so you gotta be quick!

Once you're back on solid ground, make a dash for the gate. Shoot fireballs at the two beacons to light them up. This will open the gates, allowing you to head towards the deep lake.

Now, dive into the lake and swim straight down. It's pretty deep, but don't worry, there are plenty of bubbles to give you air. You'll see a big Star Bit arrow pointing the way. Follow it to a switch that opens a gate underwater.

After that, swim back up. In the top left area, you'll find two Jammyfish guarding another switch. They move super slowly, so you should have no trouble flipping this switch quickly. This opens the gate above you, letting you swim to the final area of the level.

In the final area, make your way over to the ghost ship. Watch out for the Boos as you approach it.

Once you're on the ship, the boss fight against Kamella begins! She'll be throwing fireballs and green shells at you. Your main goal is to grab those green shells and throw them back at her. The best way to do this is to jump and then throw the shell in her direction. She likes to disappear and reappear in different spots, so try not to throw your shell too late.

Hit her with two green shells, and she'll move up to the crow's nest. Use the pole to climb up there.

Kamella will have spawned two Magikoopas in the crow's nest. They shoot fireballs, so take them out quickly before the area gets too crowded.

You'll need to hit Kamella with two more green shells to finally defeat her. Once she's down, a Power Star will appear!