|Bowser Jr.'s Robot Reactor|
Right when you land on the planet, Bowser Jr. will activate his large robot Megaleg, which will attempt to stomp on you with his huge feet. Wait until one of the feet crashes onto the planet, then quickly go on top of it. The arrows are pointing up, meaning you can climb up it. While climbing up, Bullet Bills will charge your way. Dodge them and continue on. Up ahead of these canons are rotating cogs. Jump over them and land on the other side.
Eventually you'll be on the robot's head. It's being powered by the Grand Star in its casing, and to protect it a set of fences will arise. What you have to do here is direct the onslaught of Bullet Bills so that they hit the fences. After this, quickly get the Bullet Bills to break the casing so that you can get the Grand Star.