Subclasses raise your stats by a huge amount if you master them all.
To use these extra stats you first have to Reincarnate. Subclasses are the key to a high-stat max. level character.
What you need:
It is highly recommended that you do this in the Postgame with an "Land of Carnage" aka. LoC item, which is obtained by unlocking the Land of Carnage and obtaining one there. Doing this with a normal Item makes the process WAY slower. An LoC Item is glowing red, you will notice the difference immediately.
This guide is mainly for the Sage, because only she has the Geo Blast ability which is needed for this to work.
First up, go to the Cheat Shop and set the enemy Strength to 20/20 Stars (the max. amount).
Then go into the Eclair or other LoC Item and try to find a Geo Symbol. Use your Sage to destroy the Geo Symbol with your "Geo Blast" Ability. 'Geo Blast' destroys a Geo Symbol instantly, no matter the Health or Resistances it may have. This will give you A LOT of Experience towards your subclass. You may need to destroy a few of them to max out a subclass. The later subclasses need more than one Geo Blast to max out.
After destroying the Geo Symbol, use a Mr.Gency Exit to leave the Item and maybe change your subclass if it's already mastered. You can create a lot of Exits by using the Alchemy, if you already unlocked it.
This may seem slow, but it is the quickest Method I know. After mastering every subclass, go to the Strategy Assembly and Reincarnate your character. There you will see that the base stats for Level 1 are way higher than before. This is all thanks to the subclasses you've just mastered.