Megaleg is incredibly large and lives on a planet just big enough to hold it. It has three long legs, like a tripod, and each leg has various tricks and traps designed to trip Mario up. It has a Snifit-like face and fly-like eyes with huge lights beaming out of them. On top of Megaleg's head is a red cage with a rotating lighthouse-like light on top that contains a Grand Star. Megaleg is much bigger than Digga-Leg, a similar boss encountered in Super Mario Galaxy 2.
When Mario enters Bowser Jr.'s Robot Reactor, Megaleg is positioned on the bottom of the Robot Reactor. When Mario uses the Launch Star on the Starting Planet to reach the Robot Reactor, Bowser Jr. appears and orders Megaleg to attack Mario. Megaleg mainly attacks by consistently firing multiple Bullet Bills from various locations on its head, legs, and underbelly. It also tries to stomp on Mario while he's on the Robot Reactor. Mario must jump on one of its three huge feet and climb up its leg, avoiding Bullet Bills along the way. When Mario reaches Megaleg's head, he has to lead a Bullet Bill into the larger cage surrounding the red cage, after which a barrier forms around the red cage and more Bullet Bills are fired. Mario must then lead another Bullet Bill into the red cage to release the Grand Star trapped inside and defeat Megaleg.
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