Kingdom Hearts: Chain of Memories/Sora/Twilight Town

When you get back to Castle Oblivion from Hollow Bastion, a member of Organization XIII will be waiting for you. It's Vexen, who, as suspected by Sora, is making Riku evil. There is little conversation before the time for battle comes. Vexen is a tough battle, namely because of his two health bars and his shield, which he constantly has raised. Don't even bother trying to attack directly while he has his shield up. Use the sleight of combined Cloud cards and keep trying to get behind him, since he doesn't seem to be blocking his backside. Vexen's attacks include: Once you've defeated Vexen, he'll escape and the scene will switch to Organization members Larxene, Axel, and Marluxia having a conversation. The discussion seems to be about Vexen. Once this cutscene is done, go up the stairs to see a new world - Twilight Town.
 * An attack in which Vexen swings his spikey shield at Sora.
 * Vexen can also cast Blizzard.
 * Vexen uses ice to stop Sora in his tracks. While Sora is frozen, not only can he not attack or move at all, he also loses health. This is the attack to avoid.
 * Another Blizzard-like move, though this one plays out more like a Sleight. Basically, Vexen creates a blizzard.
 * Ice shoots up from the ground at the spot where you're standing. This attack can easily be avoided with dodge roll - the ice doesn't rise quick enough, and will only create an icy path behind you.

Twilight Town
When you first arrive at Twilight Town (after the cutscene), you'll find that originally weak Heartless are at much higher levels than they once were. Nevertheless, kill all of the Heartless in this room and strike the door. Synthesize using a card with a value of 1 or more and you'll enter Room 2. Go to the south door in this room and synthesize using a Moogle Card if one is in your possession. If not, kill all of the Heartless in Room 3 like you normally would and synthesize at the door to make a value of 8 or more. In Room 4, strike the door and synthesize using cards with a value of 9 or more. Get rid of all the Heartless in Room 5 and then backtrack to Room 2. Go to the door at the north of the room and synthesize using a card with a value of 7 or higher. In Room 6, go to the north door and synthesize using a card with a value of 1 or more. In Room 7, after clearing the room of Heartless, strike the door and synthesize using a card with a value of 5 or more, a card with a value of 5 or less, a Magic Card, and the Key of Beginnings. After the cutscene, you'll face another fight with Vexen. Use the same strategy and he'll be gone for good this time. You can now leave the world.