By attaining 1000 points in the first Challenge, all of the game's Challenges will unlock simultaneously.
Boss rush
To unlock Boss Rush, clear all of the Challenges in Challenge Mode.
Challenge mode
Play 15 matches in Classic, Anniversary, and/or Concept Mode.
Concept mode
Play 5 matches in Classic and/or Anniversary Mode.
Flying shady head
On the How to Play screen, left-click the Space Bar icon 714 times with the mouse. Shady's head will appear and bounce around the screen, collecting all of the power-up icons shown.
All challenges at once...