Since this is my favorite fight in the entire game, I know an easy strategy that I picked up from my own experimenting and watching speed runs.
Items Needed:
1) You don't need to be level 50-60 unless you have a really, REALLY hard time with boss fights. Anything from level 40-45 should suffice for Beginner or Final Mix(I normally get to Riku/Ansem at level 42 or 43) and level 45-50 on Proud, unless you are an expert at Proud mode.
2) Important abilities: Ars Arcanum, Glide, High Jump, Dodge Roll, Guard, Counter Attack, Leaf Bracer(unless you were fooled to choose the Dream Sword at the beginning of the game because seriously, Leaf Bracer saves your butt a lot of the time), Sliding Dash, MP Rage, and MP Haste
3) You may also want some Elixirs, Ethers, and Hi-Potions for healing and MP needs, though also have Cura as X on your magic quick selection screen.
4) Beat the Hercules Cup first so that you can use the Olympus Keyblade, a very useful Keyblade for the battle.
Battle Tactics:
1) Open up the battle by getting close to Riku and use Ars Arcanum (gotta be quick about it) and use Ars Arcanum when Riku is staying fairly still/close to you and NOT using his dark powers.
2) When Riku throws the dark disks at you, guard to hit them back at him.
3) Now this is the part of the battle that kills a lot of people, when Riku uses Dark Aura, jump up(High Jump is gonna be a real life-saver) and glide around the field. If you are about to hit the ground, head towards the edge of the battle area and use Dodge Roll if needed and then to another Glide.
4) Anytime you are able to attack Riku and he gets too far away, either Dodge Roll over to him, or if you have Sliding Dash, just attack him and you'll just do a Sliding Dash over to Riku(the ability Vortex can prevent Sora from doing a Sliding Dash sometimes though).
5) If you ever get low on health, you can always use Cura right away without worrying about being hit as Leaf Bracer will protect you from being hurt while healing yourself. If you chose the Dream Sword at the beginning of the game, however, then you won't obtain Leaf Bracer until level 69, thus this will only help if you chose Dream Rod or Dream Shield at the beginning of the game. If you don't have Leaf Bracer, Dodge Roll or Glide away from Riku and his attacks and put on a really quick heal(and heal again if need be).
6) Put on any MP Rage and MP Haste abilities you have so that you can quickly regain MP allowing you to continue using Cura and Ars Arcanum without having to use an Ether or Elixir.
7) If you are really good at guarding, when ever Riku is attacking you with his sword(this is the normal point to also be attacking him and using Ars Arcanum) guard his attack and he'll be knocked back for a second and then quickly use counterattack after which he will attack and you must guard again, then rinse and repeat. If you can't guard or counterattack very well, then you just continue to attack him. The reason for this strategy of guarding is it's safer for you, but slower.
That's really all there is to the battle and all you need to know to survive it.