The Grandmaster Galaxy is the final galaxy of Super Mario Galaxy 2, located in World S. It is analogous to the Grand Finale Galaxy from Super Mario Galaxy because the player must collect all 240 Power Stars (without any Bronze Stars) and Green Stars to unlock it. It contains two Power Stars: one for completing the level and another one for a second completion under Daredevil Comet conditions. In the latter case, the player gets the Star from Rosalina. This is the only galaxy where Green Stars do not appear because the player has to collect all of the Green Stars in the game in order to unlock this galaxy.
If the player collects all Stars and then completes this galaxy, a special message will appear on their Wii Message Board, both congratulating and thanking the player for completing the game 100%.
This galaxy starts a trend in future 3D Mario titles of a challenging level being unlocked after completing everything else in the game (or otherwise meeting certain post-game unlock criteria), inspiring Special 8-Crown in Super Mario 3D Land, Champion's Road in Super Mario 3D World, and the Darker Side in Super Mario Odyssey.
NOTE: Unless otherwise noted, all names are unofficial.
In the first area, Mario has to use Yoshi to swing on the flower grapples to reach the Launch Star at the end of the area. There are several obstacles along the way, such as Mines and Bullet Bills. Paragoombas appear in The Ultimate Test and are replaced by Choppahs in The Perfect Run. A Toad can also be found on this planet and will shout Yoshi's name when spoken to.
After using the Launch Star from the first planet, Mario lands on a planet with Flipswitch Panels. It has a backdrop showing moving sprites of items from Super Mario Bros.. Mario has to activate all 21 Flipswitch Panels while avoiding Sentry Beams to reveal the Launch Star.
Electric Maze Area
After landing, Mario finds a Cloud Flower and a fan producing an upward gust. Mario has to use the Cloud Flower to maneuver to the Launch Star at the end of the area, with more Cloud Flowers subsequently being supplied along the way. There is also a 1-Up Mushroom at the top left which can be reached using the Cloud Flower.
This area contains planets made of shrinking platforms and Red-Blue Panels. Ota Rocks and Ring Beamers are also found in the area. Mario has to avoid all the obstacles in order to successfully reach the Launch Star to the next area at the end of the section.
Pull Star Area
In the beginning of the area, there are some electrified blocks followed by a long segment of Pull Stars, which Mario must use to reach the Launch Star while avoiding electrical currents, mines, and Paragoombas. The three figures from Shiverburn Galaxy are visible in the background of this area.
Hammer Bro Planets
The ending platform has three Boomerang Bros., which Mario must defeat to reveal a Power Star (or a Launch Star in the Daredevil run). Note that defeating the Hammer Bros. is not necessary for getting the Power/Launch Star to appear.
The Comet Medal for this galaxy is located on the far right side of these planets, next to some platforms with some torches (as shown on the screenshot). In The Perfect Run, the Comet Medal is replaced with a 1-Up Mushroom.
This area is only accessible during The Perfect Run. It is a small planet with a small house and strongly resembles the Gate in Super Mario Galaxy. Rosalina and the final Power Star are found on this planet. There is also a 1-Up Mushroom behind the house. If Mario falls off the planet, he loses a life and has to go back to the start.
Prima Official Game Guide description
Names in other languages