At the start of the battle, a dark energy will surround Roxas. Then, the screen will fade to white, and Roxas will be trapped in the air. When Flailer approaches and is about to attack, use the Reaction Command to break free and attack him. Then, Flailer will rush up and grab you. Use the Reaction Command when it flashes to knock him off the platform. The Station will turn upside down and Roxas will slide off, using his Keyblade to hang on. Flailer will generate a sphere of dark energy and launch it at you. As it approaches, use the Reaction Command and Roxas will throw his Keyblade through the attack and knock Flailer down. Next, target his head and start attacking him before he stands back up. When he stands up, he will begin to do various attacks, which includes summoning smaller enemies. Use the Reaction Commands to reach his head, and try to avoid his attacks. Eventually, his attack pattern will reset and you will go through the Reaction Command sequence again. If you take a lot of damage, make sure to use your potions.