I have heard many complaints from people saying that the final battle with Riku is way too hard and it has taken them months to complete, I also know this to be true because I have seen my cousin literally spend three months trying to beat this boss. There are several encounters with Riku, but this particular one happens right after the fight with the dragon, so if you are stuck in an earlier battle then may god be with you.
First of all, let's get you loaded up shall we. In this fight you will be alone, you won't get any help from Donald or Goofy, so you shouldn't waste any Mega items or concern yourself with their load-out. The setup I used was 1 elixir, 3 ethers and the rest were Hi-Potions. You may want to put on some darkness protection, but personally I didn't need it (nor have anything that would give it). I used the Keyblade from Neverland, but the Olympia Blade from the Hercules stage is highly recommended. Furthermore it would be wise to equip Scan, Glide, Dodge Roll and the ability that lets you jump higher, anything else is not really necessary, but counter-attack can come in handy.
The first part of the fight is pretty easy as long as you keep your health up (if your health nears the 40% it is time to heal up). Don't waste your health and Aero during this phase and play safe a lot. The easiest way to harm Riku is to stay out of his way and lure him into doing his jump move, than quickly roll out of the way and strike once or twice (preferably once) at him while he is on the ground. The problem is that Riku very often attacks you mid-combo and cause a bit of damage that is very hard to dodge. You can also attack him right after he does his Sonic move or his spin-attack, but this is definitely the safest way to reserve health.
When his first health bar is down or about to go down, Riku will start to channel electricity around his body and this is where things start to get ugly. When in this mode Riku's attacks have been enhanced: His sonic move will send out a projectile that you must dodge, his jump move will now send out an even bigger shock-wave and his large jump-attack will both signal the end of this phase and cause fiery pillars to emerge from the ground (in the same fashion as Cerberus). Keep dodge-rolling and refrain from damaging him, start using Aero at the start of this phase and keep healing and you'll make it through. After a while he will return to normal and you can resume your old tactic with the addition that you can now do some free damage whenever he is starting to go into this mode again. If you are running low on MP you should consume an ether or an elixir because Riku isn't going to give you any mana. I never ran into the problem of dying because I had no power left to use cura, but if you are having trouble it may be worth switching a potion or two for more ethers.
This leaves his third and most difficult phase which will start when his second health bar is down or about to go down. During this phase he will start to glow, when you see it happening you should instantly go to a corner, heal and Aero up and then jump into the air and glide. Riku will charge around the room lightning fast and if you get hit you will take a lot of damage. Make sure you are not targeting Riku and simply keep gliding until he stops and does his super jump move again (which will send out even more pillars now). The twist is that Riku is now back in his second phase and I have never seen him drop back to normal again, now is the time to take chances and hope for the best. Simply attack Riku and try to get him into a corner with some combos. Once again I can't emphasize enough how important it is to heal and Aero, especially when your health is nearing the 40%, if Riku hits you enough times he will kill you before you even hear the sound-effects indicating you have low health, do not wait with it.
Riku will occasionally go back into his third phase, but keep this up and you should be fine, the biggest threat in this fight is interrupted heals ending up killing you, so it is partly a luck factor deciding your fate here. Get used to that cutscene at the start of this battle, you'll be seeing that scene quite a lot. Good luck.