So when you have unlocked the arena you will be able to perform the following glitch. When you enter the arena there are two people standing nearby of which the one on the left will offer a Survival game for one character. Select a character you want to level up and make them active. Talk to the character on the left and save the game. Exit to the 'System' menu and then advance the system date by one or two days. You will notice that when you now load the saved game the character you chose will have earned a number of Experience points. This encounter is repeatable so this process can be repeated to get as many Experience points as you desire.