Looking for a challenge?
I'm not sure it this would be considered a secret or not, but oh well :)
If you're looking for ways to make the game harder, then you should visit the Grade Shop!
The Grade Shop appears at the end of the game.
First you need to select Min HP, which will bring you to the lowest possible health when starting the game.
Then there should be an option about EXP. You need to bring that down so it's at the minimum.
Finally, when you start a new game, select Mania battle mode.