To unlock the four Nintendo World Championships levels, complete the 10 Mario Challenge mode 7 times. Complete all four to unlock the Luigi Mushroom item, which can be used in Course Maker by grabbing and shaking a Super Mushroom. It can only be used in Super Mario Bros. levels.

Unlockable	How to unlock
Luigi Mushroom	Complete all four NWC 2015 levels.
NWC 2015 Levels	Complete 10 Mario Challenge 7 times.