The Boomsday Machine is a castle-like tank that Bowser Jr. invented that appears in the game Super Mario Galaxy 2. It has three Fire Bars circling around it and the top section has a a spinning-like mechanism. It is fought in the level Bowser Jr.'s Boom Bunker. The music heard during this battle is the same as in the Bowser Jr.'s Airship Armada battle. Its name is a portmanteau of the onomatopoeia boom and the word doomsday.
There are ample Cloud Flowers around the area to grab. The player must use Cloud Mario's ability to ascend the battlefield and get to the top of the Boomsday Machine. For the first round, they must run for one of the updrafts and spin to create a cloud and ride it upwards. From here, he can traverse across the various clouds scattered throughout the battlefield, using his cloud-making ability to assist him. Once they reaches the top of the tower, they must damage the machine by Ground Pounding the glass.
The second round is much like the first, but now the tower will attempt to vacuum the area, pulling the clouds out from under the player's feet. They should still be able to just long jump to the top of the tower in the same manner. The player also needs to watch out for an electric shield that covers Bowser Jr.'s dome periodically when he is sucking up Clouds.
In the final round, the tower goes mobile. This makes the player's climb to the top more difficult, since Bowser Jr.'s cockpit is much higher up. They still need to avoid coming in contact with the yellow projectiles, the spinning fire wheel and the vacuum attack. The fire on the Boomsday Machine will also destroy the player's clouds.
Once the player reaches the top of an updraft, they need to use the Cloud Flower refill to get to an even higher cloud tier. They should use long jumps to get to the distant cloud and they can score another refill. From here they can reach the dome with their cloud platform recharge. Once the player arrives at the glass dome, they must Ground Pound it a third time, which destroys the machine and reveals the Grand Star.
Mario faces this boss twice, with the second match being under a Daredevil Comet's conditions.
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