This is unlockable by just playing the game but if you want to get it before completing the main game, you can go to Sotenbori's Battle Arena which it unlocks in chapter 12.
*(optional) Buy some kiwami drink from the vending machines that has it (you can only store 5 in your inventory BUT you can store 500 in your item box, you can access your item box from the battle arena)
*Go to the battle arena and fight your way up, there are 30 floors and a checkpoint VERY 5 floors. If you have the Kiwami Drink item, use it in battle with your main character.
*The floor 28 gives you 100k XP + the extra if you use the kiwami drink