This level is only available when Luigi sends a letter via Mailtoad telling Mario to come save him. This is the third level in which you’ll need to assist Luigi. In return, he will give you a Green Star. This will likely be the last green star (of which there are three to collect) and after you’ve saved Luigi you’ll have access to the Planet of Trials. The other Green Stars can be collected in the following levels: The Secret of Buoy Base and Treasure of the Pyramid.
Collect the Pull Star Pieces
This level begins within Battlerock Barrage so you’ll be repeating what you did throughout that level during this one, too.
Your first task during this level will be to collect all 5 of the Pull Star Pieces found on the small flying saucers. There will be some on the topside and underside of the saucers. Once you have those, you’ll be able to use the Pull Stars to pull yourself into the Launch Star that will take you to the next area.
You’ll then be transported to a small saucer with a purple jump spring. Use that to get to the saucer above you. Walk to the topside of that saucer and you’ll find a blue screw. Spin on it using Y or shaking the Right Joy-Con to screw it. When you do this it will activate the saucer and you’ll begin the main challenge of this level.
Your next task is to stay on the flying saucer and avoid the cannonballs being blasted at you. You’ll be able to move to the bottomside of the saucer if you need to avoid the cannonballs, but be careful not to hit the electric balls that are rotating around the saucer’s edge. In addition to the cannonballs, you’ll need to move to opposite sides of the saucer if there are electric fences in your way. It is relatively straightforward but some sections may require quick movement to move down to the bottom side of the saucer and then quickly moving to the side to avoid an incoming cannonball.
Guide the Bullet Bill into Luigi’s Cage
Now, this is where this level diverges from Battlerock Barrage. Instead of guiding a Bullet Bill into the Power Star’s cage, you’ll need to guide it into Luigi’s cage. Luigi is found trapped beneath the saucer that the Power Star is found on. Guiding the Bullet Bill to the underneath of the second saucer can be tricky as it’ll explode if it comes into contact with an obstacle or the ground. Makes sure you guide it slowly to the edge of the first saucer first and then take it down and across to the second platform. Make sure to avoid the metal pillars surrounding Luigi’s cage and you should be able to get the Bullet Bill to crash straight into the cage.
When you free Luigi, he will give you a Green Power Star. You need three of these Green Stars to unlock the Planet of Trials.