Boss: Riku (2)
This one is incredibly hard. Well, start the battle off by casting Aerora on Sora, and try to keep it that way. If you have the Guard ability equipped, use it wisely and deflect Riku's attacks. He will be stunned momentarily, giving you a few quick hits. If you find you need to heal, fly away from him and cast Cura. If he goes berserk, then you should fly around the room until he calms down. If he starts throwing his keyblade at you, you can hit it right back at him to cause damage, but this is risky. It's best just to avoid this attack.
There is a certain pattern that you can get into that lets you take enormous chunks of life from Riku. After using Guard to block his jumping attack, you'll have just enough time to pull a full combo on him. After your last blow, immediately block as he jump attacks you again. This way, he won't be able to hit you, while you take big amounts of his HP away. If you're lucky, you will be able to hold this pattern for quite a while.
Also, take note that the faster you finish him off, the less likely he will be to stay in his powered up form permanently. This is definitely a great thing to remember.
If you can pull off using Ars Arcanum on him, that would be great.
EXP. 8500