Instructions
Prepare your card deck before the battle. Stock several Fire cards, including your Mushu summon. Your Cloud summon also comes in handy, as well as special attack such as Sonic Blade. You also need cards with a value of zero to block his attacks, some physical attacks with Keyblade cards and cards with healing items and spells.
Arrange your deck so that you have like cards in a row to produce sleights. For example, three Cloud cards in a row give you the ability to use the Omnislash combo, while three Fire cards allow you to case Firaga.
Dodge Vexen's attacks by jumping over them or maneuvering around them with the control pad. When Vexen begins to cackle it means he is about to cast a spell to freeze Sora in a block of ice. Run out of the way to avoid the spell. If you are frozen, continually press the buttons on your directional pad to break free.
Run behind Vexen after he has left off an attack to use your Keyblade cards. Do not use these attacks in front of Vexen, because he will block it with his shield.
Cast fire spells at Vexen after dodging his attacks while you are facing him. Because Vexen is weak against fire, these spells damage him despite his shield. Summon Mushu to attack Vexen with four fireballs each time.
Summon Cloud or use the Sonic Blade attack to attack Vexen when he is defending behind his shield. These attacks also ignore his shield and deliver a large amount of damage.
Play your zero cards whenever necessary to block Vexen's attacks.
Heal yourself at the end of every hand before reloading your deck. This way your hit points stay up instead of having to navigate through your deck when you need to heal. If your health becomes dangerously low, heal yourself immediately, then reload the deck.
Continue to use this pattern of dodging, attacking and healing until you have defeated Vexen.