Kingdom Hearts
 Riku (Duel)
Sora may fight Riku on Destiny Islands before reporting to Kairi for the first time. Riku is the most difficult challenge 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 kicking 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 player 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. 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 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 are 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 MP, 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 (unless you equip Blitz); Riku has fast recovery and can unleash devastating counterattacks. If you have equipped Ars Arcanum, save it for later. Also beware: magic only makes this boss counterattack with Dark Firaga.
After you drain his first life bar, 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 at a safe distance, but not too far, or Riku will make a high leap and attempt to stab you with his Keyblade, creating shockwaves that move sideways from his position. Now it will be a good time to use Ars Arcanum, but just use it a few times, and try saving your MP, because you'll be needing it later.
Eventually Riku will invoke himself with darkness, and a purple aura will surround him. During this period, his speed and agility increase, and he gains two new attacks; one involves throwing his Keyblade at you, similar to Strike Raid (be careful when countering this attack, as if it hits Riku he will use Dark Firaga), and his infamous Dark Aura. Fortunately, his thrown Keyblade can be deflected either by guarding or striking with perfect timing; for each successful deflect, Riku will receive damage and be momentarily stunned. 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, so that he no longer taunts you and instead 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 your distance from him and High Jump right before he starts shooting around. Immediately execute Glide and move around in a circular fashion. Near the end of his assault you'll have to repeat the phase and start Gliding again. 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. However, he will Strike Raid and combo you immediately afterward.
When Riku's health reaches critical levels, be sure to finish him as fast as you can, for he will 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 at him and you'll eventually win. Suggested Keyblades to use are the Olympia and Metal Chocobo—but ultimately, no matter what Keyblade you use, fight Riku tactically and you will take him down.
A good pattern to get into that causes great damage is immediately after executing a full or half combo, spam Guard, and repeat, almost never giving him a chance to attack.
 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, dark magic, 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.
 Kingdom Hearts 358/2 Days
 Mission 74
Riku will block almost all of Roxas's attacks, so the point is to simply attack with Keyblades or area spells like Aeroga in order to herd him to the southern tunnels. While several Heartless will appear throughout the area, just ignore them and focus on Riku. Once Riku is chased into the southernmost tunnel, he will escape through a Dark Corridor and the mission will end.
 Mission 93
In this battle, you will lose your Dual-Wield power, only left with the Oathkeeper in hand whilst Riku wields Oblivion. Riku is particularly tough to defeat due to his high HP count. His normal attack consists of a flurry of stabs, finishing it off with a mighty thrust that comes out rather fast though this can be blocked fairly easily and a counter attack can be used (if the panel is equipped). Riku possesses several special attacks which are signaled through body gestures. If he raises his arm up, he will cast Dark Shield, which briefly raises up a barrier in front of him. Attacking him while the shield is up will cause him to immediately counter you with a quick thrust. If he jumps back, he will cast Dark Firaga in which he shoots forth a blue energy blast. Unlike the previous games, Dark Firaga will home in on you. It moves fairly slow however, and can be easily guarded and reflected. If it connects, he will immediately follow up with Helm Break. At times, he'll perform Dark Break, where he leaps into the air and teleports to your location and strikes the ground, releasing a radius blast of energy, which he repeats two more times. This can be avoided with Dodge Roll or more effectively by Gliding.
Once Riku's health starts to run low, he will gather up power and infuse his Keyblade with dark energy, increasing his range and power. Here he becomes much more aggressive in combat, immune to flinching and will begin to utilize Dark Aura, charging you with powered thrusts from all directions and ending it with a downward thrust that releases small bursts of energy around him. Usage of Cure, Cura or Curaga and your Limit Break will prove most effective in this battle.