This is getting on person's fighting style to the maximum level (9). To do this, simply play through the game and focus on one particular style. For me, I played through a lot as Kilik. To further help, create a character using the same style. So, I played as Kilik and I also made a character using Kilik's "soul."
The experience is shared. So, if you play as a character (ex: Kilik) and get it to level 5, and make a character using his soul, you character will have a level 5 in that style. Level up your custom character to level 6 and the original character will also be at level 6.
To "boost" for experience, play the Tower of Lost Souls and make teams of the same character/same soul.