Instructions
Play through the game until you reach the final boss fight with the Phantom Virus. He moves around the room in a figure-eight pattern. When he stops and flips, this signifies an impending charge attack, so jump out of his way to avoid it.
Avoid five charge attacks and the Phantom Virus stops to laugh. A flashing "CHANCE" button appears at the bottom of the screen -- indicating that now is a good time to attack him. Press "Square" to throw a pie at him. He then switches to a lightning attack, so run around the edges of the room to avoid the bolts. Phantom Virus then splits into three versions of himself.
Avoid the charges of the three different versions of the Phantom Virus. Press "Square" to throw a pie at any one of the three versions, when you see the flashing "CHANCE" message. Phantom Virus forms into a large version of himself, after you hit him with a pie.
Dodge the five attacks from the bigger version of the Phantom Virus. He charges at you and punches down with his right hand, so jump to his left side to avoid the five attacks. Press "Square" to hit him with a pie when the "CHANCE" message flashes again, to defeat him and end the game.