Have Cura in your L1 list for quick, easy access and plenty of Hi-Potions and maybe some Ethers. Watch Riku closely to see what he'll do. Most of the time, you can just rush up and attack, attack, attack. Other times, you can jump away from his attacks (or dodge roll-left/right and square) and hit him afterwards. Early on, you sholud've gotten a skill that lets you hit four times instead of three. The best moves are jumping attacks because if a ground attack knocks Riku away, he'll hurt you. In the air, you follow your attacks.
In your skills section, focus mostly on non-MP using skills, but it's good to have a number of attacks in. so you can get the most hits in each time.
My biggest piece of advice is to hit fast and hit hard. Do as much damage as possible, so when he starts pulling out his good moves, you won't have too much more to beat him down with.
At some point, Riku will do a teleporting slash, darting all around the room. When he starts glowing, jump up and glide around for a bit to evade all the attacks, but you probably can't evade the multiple explosions that follow.
Cura on the L1 list is good for a quick healing spell but only use it when your HP is really low, and when Riku starts glowing, heal quickly. Items are also good because they can't be interrupted.
It takes practice, but you'll get it. Good luck.

Heres one way you can beat Riku (ansem Form.) All you have to do is use Guard if you equipped it Then when he starts to thrust at you, use guard immediately, then Counter. He will be hit, and if he starts to spin around, he will start to thrust again. guard at the right time, then Counter again. Repeat to win the battle. NOTE: you have to move away from him before you can guard perfectly.

How do you beat Riku (or Ansem) in the Great Hall?

level up alot and use cure and aurora alot. i suggest being at level 48 at least. he takes a while to defeat since he keeps shooting from the walls or whatever. just use glide or high jump when he does it to avoid that attack. just cure yourself right before your health thing starts going off. if you've learned second chance, equip it. it'll help alot. and if you've learned mp rage (or something like that where you gain a mp whenever your hit), equip that too. just attack 3 times then cure (unless he didn't hit you, or you have alot of hp left, don't). use aurora whenever it dies out. you can get a stronger one in never land. it's in the room that you have to do the yellow trinity to. your aurora will be able to attack and defend.