From the Super Mario Wiki, the Mario encyclopedia
Dream Spacewalk is a Dream Event that appears exclusively in Mario & Sonic at the London 2012 Olympic Games. It is set in "Synchro Battle Galaxy", a place based on Sky Station Galaxy from Super Mario Galaxy 2 and Good Egg Galaxy and Gateway Galaxy from Super Mario Galaxy. The objective is to defeat Dino Piranha as fast as possible. The starting Olympic Record is held by Team Mario at 1 minute 40.00 seconds.
Before the event starts, a group of Lumas are seen on a planet. The camera then pans over to Dino Piranha, who spots the Lumas and spits out one fireball at the planet, and two that go near the planet. The Lumas then flee and fly over to the players, asking for their help to defeat Dino Piranha by turning into Launch Stars.
After the team is introduced and the introduction is shown or skipped, the team goes into the Launch Stars and the event begins. In order to lower the health bar, the players need to tilt the Wii Remote so that the character they are controlling into one of the three Guide Rings: a big one, four plain small ones, and four character small ones, which shuffle around after being shown. After passing through the Guide Ring(s), the four characters are sent to Dino Piranha, where players can press to increase the attack. When the health bar reaches yellow, a cutscene of Dino Piranha getting angry and shooting fireballs plays. If no custom music is set for the event, music changes from "Sky Station Galaxy 1" from Super Mario Galaxy 2 to "Dino Piranha" from Super Mario Galaxy during this time. When Dino Piranha gets angry, it fires fireballs sooner and in threes, where if the player(s) take too long to pass through the ring before it gets angry, it shoots a fireball at them. If a fireball hits a character, the character is stunned for a second, and they do not interact with the Guide Ring. But, if the player press before the fireball hits the character, the fireball goes back to Dino Piranha, damaging it and making it turn away from the players for a bit. In this state, the players cause more damage by hitting Dino Piranha's tail after passing through the Guide Ring(s). After Dino Piranha's health bar has been depleted, it falls over, screams and explodes in a cloud of purple dust and the event is finished. The player's time is then shown.
The Counterattack challenge is achieved by spinning Dino Piranha's fireballs back a number of times in this event. The criteria for unlocking each achievement are as follows:
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