To unlock different secret stages simply defeat the characters that appear on them. (gamerankings.com)
Visiting shops for tips to find secret stages and shortcut paths will be a must too! (gonintendo.com)
After my first run through the game I had collected less than 4,000 and with only a few secret stages to be found and played, this will be a huge grind to achieve. (thexboxhub.com)