Phantom Ganon cannot be defeated unless all of your Warriors work together. If you try to fight the fiend before everyone arrives, he never reveals his weakness. Instead, he continually splits into four clones, then converges and strikes with devastating impact. Cia can’t withstand this punishment for long.
Once all of your Warriors gather near Phantom Ganon, they combine their power and blast the fiend with a bolt of pure light, significantly weakening him. Now’s your chance to turn the tide!
Once you’ve weakened Phantom Ganon, he stops splitting into clones and starts blasting you with massive spheres of energy. Rather than avoiding these white-hot spheres, attack them as they approach. Doing so knocks them back at Phantom Ganon, forcing him to volley the sphere back at you. Attack the inbound sphere again, and keep volleying with Phantom Ganon until you at last manage to blast him with his own attack!
Phantom Ganon stops using the sphere attack after you take out his Weak Point gauge once. He then grabs another sword and starts attacking with both blades at once, with a multitude of combos. After finishing one of the combos, use the opportunity to get behind him and attack his back until he hits the ground and exposes his Weak Point gauge again. Continue this until he is defeated.