I did this with the ever-useful Spin Kick special move.
Picking up tapes of a song will level it up. However you need to spend mithrill rocks (the things that drop when you beat a boss) to raise the level cap of each song.
Get Spin Kick's level cap raised to 50 and then you can buy upgrades from the shop on Stage 2 to get it to level 50 fairly quickly.
This takes a bit of farming and a bit of grinding but it's not difficult.