Bowser Jr.'s Mighty Megahammer is a stage in Super Mario Galaxy 2. It is the first mission of Bowser Jr.'s Fearsome Fleet, and its completion unlocks the fourth chapter of the game, The Many Mysteries of the Cosmos. The harder version, Megahammer's Daredevil Run, can be accessed by grabbing the Comet Medal on one of the airships. In Bowser Jr.'s Mighty Megahammer, Bowser Jr. launches a second attack with his Megahammer robot.
Mario starts his trek on a path with disappearing platforms and needs to head east, where Yoshi is imprisoned in his egg. After freeing him, Mario must use Yoshi to clear the path, using a Bullet Bill to break the cage causing the blockage. After this is a Lever that must be pulled using Yoshi's tongue. They must ride the lift that gained power to continue. On their way, Mario and Yoshi come face to face with Undergrunt Gunners. These gunners will attack by using their guns to fire Bullet Bills. Yoshi can return fire by eating the projectiles and spitting them back at the gunners, though if you are quick enough, Mario can gain Star Bits that come from the remnants of the weapons that were destroyed.
Eventually, the duo will reach another platform and must continue by flipping another switch, causing a second lift to go forward. The second glass cage blocks the path, but Yoshi can clear the blockage with the same strategy as with the first: using his tongue to eat a Bullet Bill (These ones home in on their target.) and then spitting the bullet into the cage. Ahead lies a Black Hole, but Mario and Yoshi can escape the Black Hole's grasp by using a Launch Star to head to the next area.
In the next area, Mario and Yoshi have to break the cage that is imprisoning the lever by using a Bullet Bill in order to make their way forward. Swing Hooks exist here to provide a path across the gap just like in their previous appearances, but Yoshi cannot use them with a bullet bill in his mouth. The player can flutter jump to cross the gap, though this is a risky manoeuvre. Hitting this lever activates a third platform that moves on a rail. However, Skyships will intercept the platform's path, having returned to their post. Mario can walk on their deck, though. One contains a comet medal and Hammer Bro. This Hammer Bro does not have to be defeated, and if the platform leaves Mario and Yoshi behind, it will teleport behind the airships to pick them up. At the end, Lumalee appears with her Luma Shop. When spoken to, she can provide a 1-Up Mushroom or Life Mushroom for 30 Star Bits. Just afterward is the Launch Star, which leads to the ringed planet where the Megahammer makes his lair.
To beat the Megahammer, Yoshi must spit Bullet Bills at the robot's blue orbs. Meanwhile, the Megahammer attacks by slamming his hammers on the stage or creating shockwaves to shock anyone behind him. After losing one orb, the Megahammer will become aggressive. The boss can also shoot Banzai Bills from the Banzai Bill Cannon on his chest. If their impact hits one, the Megahammer's Banzai Bills and hammers can destroy the yellow and black platforms. These platforms will not regenerate after their destruction, creating permanent gaps. Each orb takes damage equal to two bullet bills to destroy. Upon their destruction, the orbs may release star bits, which can be grabbed if the player is fast enough. After all three are destroyed, Bowser Jr. must be hit as the Megahammer uses his hammer attacks. After taking damage equal to two bullet bills, Bowser Jr. retreats as the Megahammer's body falls into the black hole below and is ripped apart. A Grand Star appears, and Mario and Yoshi can use a Sling Star to head over to where it is: above the pit.
- This is the only boss mission whose Grand Star cannot be reached without items (including Launch Stars and Sling Stars). It is also the only one where Yoshi can be featured in the Grand Star celebration.