Beat all courses to unlock eggman.
Collect the five coins on each course and finish 3rd or better to unlock a bonus race. Win and get a character.
Finish 1st in the 4 race to unlock this track.
If you win ALL your fights up to metal sonic, beat him in round 1, use block in round 2 and you SHOULD be super sonic.
Note: I have never done this, but I saw it with my eyes