Super Mario Galaxy 2/Boulder Bowl Galaxy

Rock and Rollodillo
Start by taking out the three Elite Octoombas around the glass cage to unlock the Rock Mushroom. Rock Mario turns into a fast-moving rolling rock when you spin he'll keep going until you run into something or jump and press. It takes two spins to break the big purple crystal on the side that the green Toad is working on, and inside you'll find the Comet Medal and a 1-Up mushroom on the other side of the far fence. To get out of the area, head over to the metal wall, spin and run into it to knock it down. It creates a bridge to six Pinheads and a ramp behind them. Roll through the Pinheads then roll over the ramp to launch to the next planet.
 * First planet

Here's you'll find a checkpoint flag and a bunch of enemies around another glass cage with three boulders rolling around it. You need to kill all the Octoguys and Elite Octoombas to open the glass cage and free the Launch Star. As Rock Mario you can roll right through the boulders and enemies, but you'll break the wooden fence around the area. The fence will stop you, but once it's gone you have to be careful you don't go rolling right off the edge. You can also jump on the enemies like normal or lure them into the boulders' path to be crushed. When it's available, take the Launch Star to the next planet.
 * Second planet

This planet has boulders and Chomps rolling around in the mud, along with a couple Elite Octoombas and most of the Toad Brigade. Grab a Rock Mushroom and spin into the purple crystal at the opposite side of the planet from where you start. Don't try rolling into the Chomp, but nothing else can hurt you while you're rolling. If you've gotten a letter from him, you'll also find a golden Gearmo here who will give you the hidden star if you help him out. Once you break the crystal, take the mini Launch Star up to a small orange planetoid with a Shop Luma, a checkpoint flag and a Launch Star.
 * Third planet

On the wire mesh planet, you'll be faced with a Rollodillo. Grab the Rock Mushroom and roll roll into the Rollodillo's exposed tail four times to defeat it. Avoid the spines and nose, as you'll not only lose your rock power-up, but also a life wedge with each collision. He starts off by slowly chasing you and running by you when you move out of the way, then skidding to a halt and turning around. When he's past you, roll and hit him from behind. After that, he rolls up into a ball and tries to roll over you. Dodge him, and when he wears himself out he'll fall to the ground dazed. Take this time to roll into him from behind. The next time you dodge him, he'll quickly turn around and roll at you again before being dazed and allowing for another hit. In his last attack you'll need to dodge him three times before he becomes dazed and you can finish him off. When you're dodge him, just run out of the way instead of rolling so you'll be ready to hit him when he stops. Grab the Power Star and return to Starship Mario.
 * Rollodillo

Rolling Crabber Romp (Prankster Comet)
This mission requires you to bowl over all of the Crabbers within one minute. Crabbers cannot resist a Rock Mario attack, so always have Mario in his rock form. Blue Crabbers will release 1-Up Mushrooms, but they don't disappear, so you can wait until the end of the mission to collect them. This is a pretty straightforward star—take the Launch Star to the Crabber planet, grab a Rock Mushroom and start rolling. If you pass up a group of Crabbers, don't forget you can stop by jumping and pressing. You'll especially want to use this technique toward the end of the run when there are only a few Crabbers left.

C'mere, Goomba (Hidden Star)

 * Note: This star is only available when you get a letter from Gearmo.

On the third planet with the Chomp and the boulders rolling around in the mud, find the golden Gearmo and he will give you your mission. Roll into one of the sharp tall rocks to release a Goomba. Let the Goomba follow you all the way back to the Gearmo. When you finish this level, Gearmo will appear on Starship Mario and will give you a 1-Up for all of your troubles.

Green Stars
The first Green Star is very hard to reach. In the first area, do not bother freeing the Rock Mushroom and head straight to the metal bridge. Wall jump off of the red railings to get to the very top of the bridge, using a spin after every wall jump to gain the necessary height. At the very top, you will see the Green Star. Use a backflip to reach up and take it. The second Green Star can be found right in front of the launch ramp after the Pinheads in the first area. Use the Rock Mushroom to knock down the bridge and the Pinheads, then don't roll and walk right off of the ramp. Rotate the camera around to get a better view at the top and you should fall right into this star. The third Green Star is on the second planet with all the pink Octoguys around the glass cage. Go to the other end of the planet where you will find an Elite Octoomba and a Rock Mushroom. Jump over the wooden fence at the back and you will land on the Green Star.