Please clarify your questions! You are all over the place!
Oh well, For Riku, You should be at least at level 45 (thats what I was at but you may want to be higher) equip the ability Ars Arcanum-use that plenty of times and have nothing but Ethers in you items menu. Use Cure, not potions to heal yourself YOU WILL NEED THE MANY ETHERS!
When Riku shoots out of the circle just use gilde to dodge it and keep your MP high.
For the Torn Page, I don't know what to tell you. I know that there is a torn page in the Professers workshop on the bookself in Holloween Town, try looking there.
For the Hercules Cup, level up, practice fighting with heartless, stock up on ethers and make sure Donald and Goofy have potions but set there customize menu to use potions only in an emergency.