For those seeking for harder gameplay, there's a littre trick to unlock the hardest game difficulty level, called Mania (O_O): you just have to hold down A and B on the second controller on the main select screen (with 1 Player, 2 Player etc), then select and enter the Options screen (still holding A+B until the Options screen fully ends appearing).

This also unlocks 9 lives foer every Continue

Mania is a lot more hard than Hardest, enemies AI is much more aggressive and they're even much faster and with more life.

Enjoy and... good luck!