Bowser Jr's Robot Reactor Galaxy: Megaleg's Moon

Planet #1
The Launch Star is trapped in a glass cage ahead. To free it, you'll have to direct one of the nearby Bullet Bills cage into the cage to break it. Look for the Bullet Bill launcher on the exact opposite side of the planet. Wait for a Bullet Bill to launch then catch its attention to make it follow you (Bill's eyes will turn red when he's tracking you). Now quickly dart to the other side of the planet and direct him into the glass cage. To avoid being hit, either get behind the cage (forcing the Bullet Bill into it), or perform a back flip in front of hit just before the Bullet Bill hits. Now launch from the Launch Star to the second planet, where the boss lurks.

Planet #2

Boss: Megaleg

This giant tri-pod like creature isn't so much a boss, as it is a giant moving level. Navigate to one of its three legs after it touches down (but be careful that you don't get stomped!), then run up its leg. Jump over or run between the three bullet bills, then cross over the spinning gears (try to cross them by staying on their highest sections) and continue to the cage at the peak of its head.

Here you'll find the Grand Star inside a small cage, contained within a much larger one. Like before, lure one of the nearby Bullet Bills into the first cage to shatter it, getting you one step closer to the star. However, a fence will rise from the ground to block it from attack.

As you've likely guessed, you'll have to destroy part of the fence, then lure another Bullet Bill through the busted section and into the cage containing the Grand Star. However, what you may not realize is that the fence regenerates after several seconds, so you have to move fast. Ideally, try to get two or more Bullet Bills following you, spaced a few seconds apart. That way, you can direct the 2nd Bullet Bill into the cage as soon as the fence is destroyed.