Instructions
Head right at the beginning of the level and jump over the large flamethrower and into the pit on the other side. Here you'll collect the first star coin.
Continue right past the halfway point and use the ground pound move to destroy the pile of large boxes. This will reveal a green pipe.
Take the green pipe down and start hopping on the gray platform until it moves. This will take you to the second star coin, then back to the green pipe. Take the green pipe back up.
Continue towards the end of the level and you'll see the third star coin on the other side of a box. The only way to destroy this obstacle is to blow it up. Kick a Bob-Omb into the box's side, avoid the flamethrower and collect the coin. Then go down the wooden pipe on the right to begin the boss battle with Bowser Jr.
Grab onto the propeller block in the room and dodge Bowser Jr.'s fire attacks as it lifts you up. Drop onto Bowser Jr.'s head once you've floated above him, then repeat the process two more times to defeat him and open up World 5.