This one was really challenging for me at first, before I found out about using items on Ganon. Go after Phantom Ganon first, he's farthest and easiest. Hit the blue orbs back three times, or at least one of the dark ones for a Weak Point. When he has two swords, you can't block his spinning attack, so just run out of range. Then give him three hits in the back while he laughs for the Weak Point.