BoneFine Galaxy

Planet #1
Come on, do you really need help here? Use the Launch Star to land on the planet below.

Planet #2

Boss: Kingfin

Although Kingfin may look like a shark, he's got the bite of a catfish. Kingfin himself doesn't have any attacks, aside from trying to swim into you head-on. However, he is escorted by a posse of robotic fish that will try the same thing. Thankfully, so long as you stay on the move, it doesn't take much to dodge them.

Grab any of the numerous Koopa shells along the ocean floor then chuck them into any part of Bonefin to damage him - it takes five to do him in for good. If you manage to lose track of him, simply float to the surface and look down to view most of the planet.

After each hit, more robotic fish will appear. Again, don't worry about them, just tail the Kingfin and toss shells to do him in.
Tip: Look for a Mega-Mushroom on the ocean floor, near the treasure chest.