if you want to challenge a boss then do this step: You will see a purple thing dancing on a song click the song and play it. after finishing the song you will see a purple circle on the lower right. If you get 3 purple circle click the song with the purple dancing again there will be a boss.
 "SCGMD4"){kind=icon}
 "Everybody Edits"){kind=icon}
 "The Company of Myself"){kind=icon}
 "Continuity"){kind=icon}