Strategies
Kingdom Hearts
Riku (Duel)
Sora may fight Riku on Destiny Islands before reporting to Kairi for the first time. Riku is the more difficult challenges on the island. Riku's attacks are similar to other battles with his normal strikes. He has two special attacks. If Sora knocks Riku down Riku will jump back up slamming into Sora if the player does not have Sora jump out of the way. His other comes if Sora hits him while he is guarding three times he will unleash a powerful strike though this can be countered or dodged. The difficulty of this battle comes from falling off the small battle arena (if the player goes out of bounds the play will lose.) Riku is worth 5 EXP.
Riku (Soul Eater)
Here, you will be reunited with Donald and Goofy (after playing with only one party member, the Beast, for quite a while). The Beast will not join you in this party, so make sure your Strength and Defense are up. Like Oogie, Riku can’t be hurt by magic, so use Dodge Roll to avoid lunging attacks, then counterattack him from behind.
His moveset is about the same as the battle in Destiny Islands, but now that he has the Soul Eater with him, they can be very damaging. He moves very fast too, so you may either Guard or Dodge Roll out of harm's way. Melee attacks are recommended for this battle - and like all other boss battles, heal when necessary. Good evasion, good timing, lots of physical attacks - these are sure to get Riku down rather quickly. If your stats are rather low, however, this can be a tiresome fight.
You may consider summoning Tinker Bell to restore your HP periodically.
Riku (Keyblade)
Here, you'll be fighting a possessed Riku. Before approaching him, re-stock Sora's items and make sure that he is in top notch condition. You are alone in this boss battle. It is strongly recommended that you equip Guard, Dodge Roll, Glide, and Counterattack. Also, it is highly recommended that you beat the 3 cups at Olympus Coliseum and get the Olympia Keyblade, and if you don't mind losing some magic, the Metal Chocobo can be useful as well. Riku's HP is made up of several bars. In his first HP bar, he'll attempt to toy with you, passing around you and executing his standard two-hit combo; just simply dodge and attack him. Whatever combo that you use, try to refrain from executing the combo finisher, like all incarnations, Riku has fast recovery and devastating counter attacks. If you have equipped Ars Arcanum, save it for later. Also beware: magic does little, if not nothing, to this boss.
When you drain his first life bar away, Riku will start using a new attack, he'll spin around while swinging his Keyblade, similar to Sora's vortex. He uses this attack very often, mostly to break your combos. During this time, try keeping yourself from a safe distance, by not too far, or Riku will make a high leap and attempt stab you with his Keyblade, creating shockwaves. Now it will be a good time to use Ars Arcanum, just use it a few times, and try saving your MP, you'll be needing it later.
Eventually Riku will conjure a ritual-like move, where purplish auras surround him. During this period, his speed and agility increases, and he gains two new attacks, one involves throwing his Keyblade on you similar to Strike Raid, and his infamous Dark Aura. Fortunately, his thrown Keyblade can be deflected either by guarding with perfect timing or just strike it (again, perfect timing), for each successful deflects, Riku will receive damage and be left stunned for a moment. Seize this opportunity and bash him with all you've got.
When Riku's health bar drops to his final one, he'll become more and more aggressive, he no longer taunts you and uses Dark Aura very often (watch out for the phrases "Behold the power of darkness!" and "Open your heart to darkness!"). Riku's Dark Aura is a predictable move, but it can be difficult to dodge. When you see him floating and charging, keep you distance from him and start to High Jump. Immediately execute Glide and glide in circular fashion. Eventually you'll land again, repeat the phrase and start Gliding, Riku's finisher covers about 60% of the arena, so it'll be pretty hard to evade it. Alternatively, when he charges to one particular end of the arena (he usually starts with the top left corner), jump and stay there: Riku will not charge into the same area twice. Stay there until the finisher is executed (because he will not hit that area).
Attacking and counterattacking are good tactics while Riku is in this form, just pay attention to what he says and heal occasionally and you should be fine.
Note also that aside from the first hit, he cannot use Dark Aura when you stand RIGHT next to the computer/desk/worktop. However, he will Strike Raid and combo you IMMEDIATELY afterwards.
When Riku's health hits critical, be sure to finish him as fast as you can, he'll pull Dark Aura very often, thus prolonging the fight. Aside from that, he'll start to keep his distance from you, so track him down and continue hacking him and you'll eventually win. Suggested Keyblades to use other are the Ultima Weapon and Olympia - but ultimately, no matter what Keyblade you use, fight Riku tactically and you will take him down..
Kingdom Hearts 358/2 Days
You will be fighting Riku as Roxas with Oathkeeper. Just keep attacking him. He will sometimes block with dark shield. If you try to attack him right after he blocks your attack, he will counter with a powerful slash. His combo does not do much damage; you can easily guard from it. He will also suddenly appear above you and does that three times total with shockwaves coming out. He will use Dark Firaga which sometimes homes on Roxas. After taking about half his HP, he will make his weapon a slightly bigger blade that glows. In this state he will not flinch when hit and his moves get stronger. He will occasionally perform Dark Aura on you. Just keep dodge rolling out of the way until he finishes. Keep attacking him and heal when necessary.
Kingdom Hearts II
Sora battles the hooded Riku in the form of Xehanort's Heartless in a one-on-one match in the Land of Dragons. While not a full-fledged battle, Riku attacks with slashing combos and the Dark Firaga attack. Both attacks can be dodged rather easily. He remains silent and can be damaged with the Reaction Command provided by the Rapid Thrusters that surround him. If you bring many Ethers into the battle, a good way to damage Riku is by using Trinity Limit. The battle ends when Sora deals enough damage to Riku. You could just battle the Rapid Thrusters since they are endless and try to level up.
Videos
<videogallery> Video:Kingdom Hearts: Riku - First Battle Video:Kingdom Hearts: Riku - Second Battle Video:Sora vs. Riku Showdown Video:Kingdom Hearts 358/2 Deep Dive Level Riku Fight Video:KIngdom Hearts 2 (Unknown) </videogallery>