Super Mario Galaxy 2/Bowser Jr.'s Fiery Flotilla

Star 1: Gobblegut's Aching Belly
Follow the path until you reach the large doors. Flip the switch near the area to open them. Proceed past the Dry Bones and ride the Launch Star into the second area.

Avoid the fire wheels and follow the path to the Launch Star. You will land on the area with the boss.

Boss: Gobblegut. To kill Gobblegut use your spin attack on all of the red bumps on his body. This is much more difficult than it seems. Once the boss is defeated, grab the shiny Power Star.

Items
The Comet Medal is located on top of the Whomp in the first area and is very hard to get. Jump on top of the ? block in front of the Whomp and do a backflip to land on top of the Whomp's head. Once the Whomp raises, jump and grab the Comet Medal. While you are on top of the Whomp there are still more things you can do. When on the Whomp, backflip towards the castle's wall to land on top of it. A 1-Up mushroom can be found up there. In the second area there is a Life Mushroom next to two rotating fire wheels.

Prankster Comet Star
This comet star challenge requires you to complete the Gobblegut's Aching Belly mission with only a limited amount of time. Move swiftly, do not aim to get far away items, and avoid enemies. Your success on this mission depends entirely on how well you know how to fight the boss.