Gold Leaf Galaxy is accessible in the Engine Room with 34 stars.
Star 1: Star Bunnies on the Hunt
Look for the 5 bunnies around the initial stage for the 5 blue star pieces to complete a jump star. Once active, the jump star leads you to another planet where you have to catch a bunny. It helps to ground pound the switch to elevate walls on this tiny planet so the bunny will be more easily cornered.
Star 2: Cataquack to the Skies
Lure the cataquacks to the circular launch pads with the cataquack emblems to launch yourself planet to planet. When you get to the area with the wood tower that looks like Jenga blocks, lure the cataquack to the green patch with the tiny shadow to launch yourself into a Bee Mario mushroom so you can fly up the tower with the cataquack's help.
Star 3: When it Rains, it Pours
With all the rainclouds scattering the area, Bee Mario is certainly in constant danger. Not to worry! They've placed the Bee mushrooms strategically around the stage for when you need to get on the flowers or even on top of clouds.
Make it through the stage all the way up the tower to the boss in the pivoting tank that launches water balls. If a second player is at hand, they can make themselves useful by popping the water balls before Mario gets hit. Ground pound the red switch in back to elevate panels in front of the boss for a short time. Jump on the moving platforms over to the panels and jump up to ground pound the boss. Repeat this process 3 times for victory.