Kingdom Hearts: Chain of Memories/Riku/Castle Oblivion

After the cutscene, clear the room of Heartless and strike the north door. Synthesize using a card with a value of 1 or more. In Room 2, again, clear the room of Heartless and strike the north door. Synthesize using a card with a value of 1 or more. In Room 3, after getting rid of all the Heartless, strike the north door. Synthesize your cards to get a value of 13 and use the Key of Beginnings to see a cutscene. Riku tells Ansem to show himself and come out from wherever he's hiding, but Ansem responds by saying they should join together once again. He says that since Riku accepted the darkness, they should be on the same page, so joining would only make sense. Riku rejects the offer and Ansem gets angry. He tells him that he's foolish to fight with him, but if he wants to fight, they will. The boss fight begins.

Ansem
Ansem is as tough a battle as the final boss of Sora's Story, Marluxia, if not even tougher. This battle will be much harder without Dark Mode, so use it and the moves that come with it whenever you please, but be warned; if you let him get an attack in while you're in Dark Mode, you'll transform back into your normal self as quickly as you transformed into your dark self. Avoid his attacks at all cost, whether you're in Dark Mode or not. Ansem has four health bars, so not only is this a tough battle, but a tedious one as well. Dark Firaga has the best effect on Ansem, though the shadow behind him and its protectiveness can get in the way. It will frequently jump out to guard Ansem, making it harder than it would have been without it to get an attack in. Ansem's attacks include: Once you've defeated him, enjoy the ending cinematic. You've beaten Reverse Rebirth!
 * The shadow behind Ansem, as if it wasn't annoying enough already, can also attack Riku. It's safe to say that this boss battle would be much easier without the shadow. Card break its attacks whenever you can. If not, dodge it and get behind Ansem with Riku's special dodge move.
 * A bolt of lightning is sent through the floor by Ansem. This move should be familiar from your first battle with Ansem. Avoid it like you normally would have.
 * Ansem, as if he couldn't get any more annoying, extends the battle with the use of Item Cards. He heals himself randomly. It's a good thing the items he uses aren't too powerful. Get his health back down shouldn't be too much of a problem.
 * Ansem forms a shield around him and the shadow behind him. They smash themselves into Riku. Steer clear of this move, as this one can cause a considerable amount of damage.
 * The screen goes dark and the shadow comes up from under Riku, attacking him several times. This is the only one of Ansem's new moves, and as such, it has a lot of power behind it. A card break is a good solution to this one.