This level is a dark one, with a lot of Boos and Peepas, but you will have the ability to equip a Light Box, which is essentially a flashlight. Shine it at a Boo or Peepa long enough to actually defeat them for good. Although they aren’t actually needed for any of the collectibles, it’s best to keep it on you throughout the entire level. Grab one of the Light Boxes at the beginning of the level and get on the platform ahead, which will begin moving shortly. You’ll meet a few enemies along the way, so shine the light on them until they disappear. Towards the end of the ride, you should see a ledge with a switch on it.
This is the Luigi character switch, so you will need to be playing as Luigi to step on it, which rewards you with the Stamp. Drop down to the area below and look in the mirror on the wall. There will be a collectible in the mirror, so head in that direction to grab Green Star #1 on the thin ledge. Continue onto the next moving platform, where you’ll encounter some Peepas and quite a few Parabones. The POW Blocks will hurt the latter, but not for good. There is a Red Ring here and if you collect all of the coins, you will be rewarded with a Super Leaf.
At the end, jump off and go through the door, putting you in front of the Checkpoint Flag and more Light Boxes. Get on the next moving platform, which goes up for a bit, before rushing forward. Before sprinting forward, you will want to get ready to jump off to the right, as there will be a ledge with a Mystery Box. The bonus area will require you to defeat six Boos with a Light Box within 10 seconds. Trying this individually will be a problem, so try to gather them up in one spot to get as many as you can, with your reward being Green Star #2.
Take the door to your right after exiting, with you ending up on the far side of the gap with a bunch of doors. Take the right one to reach the second floor, then take the right one again to reach the home stretch. This next platform spins a lot more and is pretty narrow, so clear out the Peepas as best you can, making it past the Parabones and Big Boo at the far side. If you can defeat Big Boo, then claim Green Star #3 without trouble. Otherwise, you can lure him out and quickly grab it to avoid damage. Exit through the door at the end, then leap from above to the Goal Pole.