You unlock a bonus level (8-Crown) by beating every single level with Mario & Luigi, and you also need to get a gold flag for each level. This bonus level's pretty damn difficult, I died around 60 times before beating it. Here's a video of it:http://www.youtube.com/watch?v=ahVLMIJ2Nxs
-Gang Starr / Nujabes blend - https://youtu.be/lsci1vu6ickDOOM Tribute - https://youtu.be/qmBQ2BDefKMDonut of the Heart cover in Javascript - https://youtu.be/afLc2CkC8lk