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

Gobblegut's Aching Belly

 * First planet

Follow the path, avoiding the Goombeetles and fire wheel until you reach the Thwomp, which has the Comet Medal on top of it. In the area past the Thwomp, take out he Magikoopa that appears, then go around the flip the switch on the side to open the doors. You can get on top of the castle walls by backflipping from the top of the Thwomp as it's rising up to give you a boost. Up there you will find a 1-Up mushroom and some star bits. You can jump directly to the small planetoid with the checkpoint from the wall, and if you go down into the area below, the double doors will open automatically if you didn't flip the switch first. Flip the switch near the area to open them. You can use the Launch Star on the steps above the Dry Bones if you go through the doors to get to the planetoid.

Here you'll want to avoid the fire wheels and follow the path to the Launch Star. Along the way you should just avoid the Dry Bones and jump on the Magikoopa as soon as it appears. Next to the fire spinners right below the Launch Star you'll find a Life mushroom, which you might want to pick up, since your next stop is the boss.

To kill Gobblegut use your spin attack on all of the red bumps on his body. This is much more difficult than it seems. While he's flying around, you can jump up and spin to break a segment, but only when his body is close enough to the planet. When he sees you, he'll charge and scrape up the earth trying to get you, so run or long jump out of the way. When he gets angry, he'll let out some steam and burrow into the planet. This is the best time to break the red segments, as they will briefly pause on the surface before going underground. When you've destroyed three of the segments you'll see a short cutscene and he'll start moving faster. Keep moving to avoid his attacks and take out the other three segments to finish him off. If you still have segments on the back part of his body, you'll need to run away from where he goes into the ground to distract him before running over and breaking them so you don't get hurt just standing next to his body waiting for a red segment to come. Once the boss is defeated, grab the shiny Grand Star.
 * Gobblegut

Fiery Flotilla Speed Run (Prankster Comet)
This star requires you to complete the boss fight from Gobblegut's Aching Belly in two minutes. The same strategy applies, but keep a watch on your health as you won't have the benefit of a Life mushroom for this fight.

Green Stars
The first Green Star is on the castle wall. Backflip from the top of the Thwomp as it is rising up to get up there, then head left and go out to parapet on the end to grab the star. The second star is in the middle of the lava on the small planetoid inside the castle. It can be obtained by long jumping out into the lava behind the Launch Star that takes you to Gobblegut, or by jumping back toward the camera from the checkpoint platform and taking some damage hopping to it.