You know that purple swirly line that passes over a song. Do that song and afterwards return to the main menu. When you go back to the song select there will be a purple ball in the lower right hand corner. Do another song that the purple swirly line passes over then you'll have 2 purple balls. You need 2 purple balls in the lower right hand corner, once you have that do a song with ANOTHER purple swirly line but this time right before you do it, a boss will challenge you. In short get 2 purple balls in the lower right hand corner to fight a boss.
After I killed the 2 guys on the first level
"Hey, you want to go to the great beyond right now?" "Because I can take you there. It's all Brimstone, smoke, and Beiber Music"
"I figured I'd go to heaven." "After what you just did? Hell no." lol