From the Super Mario Wiki
Dream Spacewalk is a Dream Event in Mario & Sonic at the London 2012 Olympic Games. The game is set in "Syncro Battle Galaxy", based primarily on Sky Station Galaxy from Super Mario Galaxy 2 but also including elements of Good Egg Galaxy and Gateway Galaxy from Super Mario Galaxy.
A group of Lumas are seen on a planet. The camera then pans over to Dino Piranha, who spots the Lumas. He spits out fireballs and the Lumas flee. They fly over to the players, asking for their help to defeat Dino Piranha. They turn into Launch Stars and the event begins.
The event focuses on defeating Dino Piranha. Players must maneuver into rings, solo or as a team, and once the ring is fully charged, they launch themselves into Dino Piranha. Players can deal additional damage by performing their spin attack after their rings are charged. After a few hits, Dino Piranha will turn orange and spit fireballs. To get a higher score, at least one of these fireballs must be deflected with a spin attack. If the spin attack is successful, Dino Piranha turns around. Once players fully charge their rings, they can slam into Dino Piranha's tail, inflicting more damage than normal.