Kingdom Hearts: Chain of Memories/Riku/Hollow Bastion

After watching the opening cutscene, you'll start off at Castle Oblivion. Select Hollow Bastion as your first destination. When you reach it, attack the Heartless you come across. Riku's Soul Eater will be able to defeat the starting Heartless with ease. His Soul Eater starts off stronger than Sora's starting Keyblade and his battle style shouldn't be too hard to get used to either, considering it's simpler than Sora's gameplay mechanics. After you clear the room of Heartless (which should get you up a few levels), go to the door and synthesize using a card with a value of 1 or more. Do the same in the next room - kill all of the Heartless and strike the door to the right of the room, then synthesize with 1 or more. Now you'll reach Room 3. Use the Key of Beginnings and an Attack Card on the door here. Riku's memories continue, with Riku being given a room by Maleficent and betraying his friends for the darkness. At this point, Riku isn't very proud of himself, and tells whoever is telling him this to shut up. After the scene, go to the north door and synthesize using a Gimmick Card.

In Room 4, once again, battle all of the Heartless and use a card with a value of 1 or more after striking the door. You should be in Room 5 now. Go to the door and synthesize using the Key of Guidance and a card with a green border. Another cutscene follows, with Riku having an argument with whoever is talking to him. Riku claims he rejected the darkness, while the other person seems to believe otherwise. Riku continuously sticks to his claims despite the other's certainty, and you can now move on. Strike the door and use a card with a value of 2 or more. You'll reach Room 6. Kill all of the Heartless here and synthesize with a value of 5 or more and the Key to Truth. Riku finds Maleficent in the next room. He's confused, since he was pretty sure Maleficent was defeated by Sora. Maleficent then explains to him that she is merely a memory, and continues on with what the other person was saying. She says Riku turned to the darkness, but Riku says he'll kill any dark person that comes in his way. He attacks.

Maleficent
This is the same boss as the one you faced in Sora's Story, albeit this one being much, much easier. She has a small health bar that shows that you clearly have the upper hand. Not only that, but she is extremely weak. You should be close to finishing her off by the time she lets out her second attack, if not her first. Keep jumping and attacking, dodging the attacks she throws at you isn't even necessary. Just keep attacking and she'll be finished with quick enough. If you do manage to let her get an attack in, these include:
 * Maleficent spits green fire all over the ground. Clumps of fire are scattered here and there. Just jump and continue the assault after she does this one.
 * Maleficent thrusts her head forward to try and bite Riku. This is probably her weakest attack, and acts as Maleficent's form of melee.
 * Maleficent stomps the ground. The force of the stomp releases shock waves, so jump over the waves to avoid this one.

Ansem
After finishing the battle, go up and synthesize at the door with 3 or more. Kill all of the Heartless in Room 7 and then strike the door. You'll see another cutscene, where the person who had been talking to Riku earlier is revealed to be Ansem, Seeker of Darkness - the final boss of Kingdom Hearts. Ansem is trying to tempt Riku to join his side once more, but Riku rejects his request. Ansem then decides he's going to use force, and says that without the darkness, Riku is weak. When Riku looks like he's about to submit, King Mickey shows up to help him out. Mickey tells Riku that light will always be in him, even in the deepest darkness - he just has to believe. Riku says he does, and, enlightened, tells Ansem the darkness will be losing today. Ansem calls him a fool and the fight begins. Again, no strategy is really required here. This boss fight is the same case as Maleficent's. It may have been a tough boss fight in Kingdom Hearts, but the fight is pretty easy this time around. All you have to do is card break. Ansem's attacks include: After the boss fight, a long cutscene will follow. After the cutscene, Riku will end up back at Castle Oblivion. Choose Agrabah as your next destination.
 * The large shadow looming behind Ansem can be made to attack Riku. This is Ansem's form of melee, same as in the first game of the series.
 * A bolt of lightning is sent by Ansem through the floor. When you see this, jump and avoid that floor.
 * Ansem and his shadow are surrounded by a shield. They proceed to attack Riku this way.
 * Ansem can also use Item Cards to heal himself, apparently. Attack him before he can bother with this.