The Boomsday Machine is the last Bowser Jr. boss battle in Super Mario Galaxy 2. Fought in the mission Bowser Jr.'s Boomsday Machine, it is a mobile castle-shaped tank that is encountered when Mario makes his way to the boss planet in Bowser Jr.'s Boom Bunker. Its name is a pun on "Doomsday" and the word "boom". It is studded with fans that can pull your clouds in, an electric field that deals damage if Mario touches the machine's outer wall and glass cage, and a cannon that fires energy balls. It and the Megahammer are tied for the biggest bosses in the game.
Mario needs the Cloud Flower to defeat Bowser Jr.'s Boomsday Machine. It has two main attacks: launching spheres of electricity from the cannon and electrifying the glass dome. When its vacuum goes on, Mario must make clouds to progress higher that eventually becomes sucked in; however, the electric balls will disintegrate the clouds. In order to defeat it, Mario needs to ground pound on the glass case with Bowser Jr. in it three times. After Mario ground pounds the glass case two times, the Boomsday machine releases threads and becomes mobile. When Mario defeats it, it explodes and releases a Grand Star as its pilot is sent flying to his doom. If you almost out of life while fighting this boss there are blocks on the ground near the edges of the ring, if you ground pound those blocks, you will get a coin. These blocks have nothing in the Daredevil Comet mission, however, as taking damage is a death sentence.
- The Boomsday Machine is very similar to the Undergrunt Gunners' cannons from the first game, from appearance, and the fact that the enemy sits in a glass cockpit at the top and get sent flying into space after the machine's destruction. The Boomsday Machine, however, resembles the Toy Time Galaxy's Undergrunt Gunner the most, since that one is even taller than the other tanks and they both shoot the electricity balls. Additionally, both can charge up their machine temporarily to prevent Mario from attacking.