Bowser Jr.’s Mighty Megahammer
Follow the two appearing floor plates and walk right to the Yoshi egg. With him you grab one of the small Bullet Bills and destroy the glass in the middle. Turn the lever and move onto a yellow platform that floats with you to the right. Take care of the obstacles and Bullet Bill. At some point you reach a second lever with which you activate the next platform. During your journey, you must swallow one of the bullets and hurl it against the glass to finally reach the Launch Star.
Touch the Checkpoint Flag and grab a Bullet Bill. Jump to the far left to the next platform and then up to break the glass case. Hurry, so that the Bullet Bill does not explode into Yoshi's mouth. Then go to the yellow platform and take it north. On the second ship you will find a Comet Medal on the left.
So jump over quickly and don't miss your record. Finally, you have to destroy the glass plate again using a Bullet Bill. Run through the Checkpoint Flag and jump into the Launch Star, which catapults you directly to the Megahammer’s arena.
Boss — Megahammer
The giant robot shoots Bullet Bills at you. Swallow them with Yoshi and send them towards the two blue circles. You have to meet them twice. Then go into the Launch Star so that you are shot at the back. Here, too, there is a blue dot, which you have to hit twice with the Bullet Bills. Just before you use the Launch Star, you can put a Bullet Bill in your mouth and shoot it right when you land. This method is quite easy, but not absolutely necessary, because you have enough shells at the back.
Every now and then, the robot strikes the platform with its two hammers, so that electric circuits escape. At best, stand in the middle and jump up as they approach you. If you've damaged all three circles, he'll do this exact hammer attack and bend down a little bit. Shoot a Bullet Bill at Bowser Jr.’s cockpit and repeat the whole thing again. Then you have finally defeated the third opponent.
Megahammer’s Daredevil Bash
Select this challenge when the Prankster Comet appears.
In this cometary star you have to compete again against the Megahammer. This time you will not be harmed. So focus very closely on the trajectory of the Bullet Bills and avoid them. If you are in the trajectory of several of these bullets, just run away and do not shoot at the robot, otherwise it can get very tight. With a bit of staying power, the star is quickly earned.
Comet Medal
When the moving-platform route passes the airships, board the second ship with the Hammer Bro. Collect the medal on that ship before returning to the main route.
Green Stars — unlock requirement
Collect all 120 regular stars and defeat Bowser again to unlock the Green Star hunt. Choose the corresponding Green Star mission from this galaxy’s mission screen. Follow the detours below rather than collecting the normal mission’s ending star.
Green Star 1
While riding the moving platform through the gunner section, use Yoshi to catch a Bullet Bill. Fire it at the rightmost Undergrunt Gunner. With the turret removed, reach the star directly behind its position.
Green Star 2
On the second airship, find the Hammer Bro and Comet Medal. Walk carefully out along the ship’s front plank, face the green star and jump from the tip towards it.
Check each numbered Green Star on the mission screen after collecting it. The routes finish immediately when the star is touched, including stars suspended over a gap.