Make Mario a Star
Here it is necessary to convert all blue question mark plates into yellow exclamation mark plates by touch. Run in straight lines across the level and make one field after another. Try not to enter so many green plates as they disappear. At the end you have to go all the way back to collect the Power Star. Use the long jump in combination with the spin very often, because you can skip many of the plates. By the way, the Comet Medal floats at the far left, which you should easily reach.
Luigi's Purple Coin Chaos
Select this challenge when the Prankster Comet appears.
Collect every 100 Purple Coins from this comet star within two minutes. Also, always keep moving, because the cunning Cosmic Clones are on your heels again. Use the long jump and spin here as often as possible in order not to make too many floor plates disappear into the sinking. Always try to collect the Purple Coins in rows in order to save you unnecessary way back. If you always run and do not miss any of the coins, the time limit should not be a problem.
Comet Medal
Search the bottom-left of the Mario-shaped starting planet. Reach the medal over the shrinking tiles, keeping a route back to a solid Flipswitch Panel.
Green Stars — unlock requirement
Collect all 120 regular stars and defeat Bowser again to unlock the Green Star hunt. Choose the corresponding Green Star mission from this galaxy’s mission screen. Follow the detours below rather than collecting the normal mission’s ending star.
Green Star 1
At the top-right of the Mario-shaped course, stand on the disappearing platform behind the hat’s lava pool and above the ear. Centre Mario on the tile and let it shrink away, dropping straight into the star below.
Green Star 2
At the far upper-left, find the disappearing platform on Mario’s right-hand section. Stand precisely in its centre and wait for it to vanish. Avoid drifting sideways during the drop towards the star.
Check each numbered Green Star on the mission screen after collecting it. The routes finish immediately when the star is touched, including stars suspended over a gap.