In chapter 7, after you defeat all of the enemies in the arena it will say that you have no one left to fight. However, if you leave your home castle and go to one of the castles you conquered, view the message that says there is no one left to fight at that castle, and then go back to your home castle, you can access the arena again. This way, you can easily get most of the second generation to level 30 in the first chapter you have them.