It definetly helps to prepare yourself for this battle, so as to save yourself the trouble of watching Ansem/Riku explain the situation to Sora over and over again. A keyblade with a longer reach, such as the Metal Chocobo, should probably be good. Keychains that extend MP won't be very helpful, as magic does not do a lot of damage. Better for this battle is elixirs. 3-5 of them should be sufficent. You can use Cura until you are almost out, for as you know, elixirs heal HP and MP. Also look for equipment that reduces dark damage, ups HP, and enhances strength and defense. Don't bother equipping items to Donald and Goofy, as you'll be fighting one-on-one. You will want Second Chance, dodge roll, glide, critical plus, and the MP abilities equipped. If you want some extra tech points, guard wouldn't be a bad idea either. Mind you, it's risky. (For those who want to take minimum damage, you better be good at using Aero!)

The Battle:
 The actual battle in itself was extremely difficult for me, but by experimenting, I've found a pretty good way to do it.

Phase One: When the battle begins, immediately lock on and whack him with a combo. You'll see that the Metal Chocobo won't do critical damage, atleast, not often enough. Almost all the difficult battles have hit and run tactics, and this isn't very different. Following the first combo, dodge roll around the arena, keeping him in the camera at all times. Get close, then immediately run away. Play hard to get until he brings his blade down in a Leon-type move. When he's getting up, this is an ample opportunity to deal a combo, or maybe even a double combo. It just depends on how fast you can get to him. He also has a thrust attack, and when you get out of the way, hopefully, he'll still be thrusting. Whack him and get away. He will also swing the blade, and right after he finishes, you'll get a double combo in(hopefully). You'll need to cure quite often, so find the best time for you. 

Phase Two: This is exasperating and annoying. Riku will glow slightly, and the first thing he will do is use his version of strike raid in your direction. He'll do this twice, and when he does, you have two options. You can just dodge away, which will leave you unscathed, or you can try guarding against it. Guarding gives you tech points, but you MUST have good timing, or it will take HP away. Another common attack is he will thrust his sword into the ground like Cloud and energy waves will fly out in opposite directions. This is also mildly easy to avoid, as long as you dodge roll away. Hit him when you see an opportunity, like just when he has finished an attack. Then, it's back to Phase One.

Phase Three: Actually, this is more like Phase Two and a half, but what the heck. After you've chopped a few bars off, you will find he goes into his "special" mode more and more often. In addition to his new attacks, he has a few newer ones. You notice in the Hercules Cup that Cloud uses the...not-quite-ragnarok attack, but it's longer. I can't remember the name, but if you do, drop me a comment, would you? Well, happy days, Riku has it too, except his's much faster. There is only one way to avoid this attack: glide. You know when this attack is about to happen, because he is suspended in the air, glows bright blue, raises his hands up, and says something that I can't quite remember. Anyway, right before he begins the attack, glide around until he finishes. Then, if need be, heal yourself. Remember good 'ol Ceberus and his dark pools that follow you around everywhere? Well, luckily for you, you get to do that again! When Riku stabs his sword into the ground, he will glow with a blackish light first, so when he does, quit gliding and start dodging. If you glide, the energy will get you.

 So as always, find the pattern and remember it. Then, develop the style that fits you best. This is only my style, the way easiest that I found defeating Riku. Heh heh...so maybe it isn't so easy. If you have any more helpful hints, feel free to post a comment, and I'll get back to you!