The Techniques-
When you’re fighting Phantom Ganon, you need to be accurate with the technique of
defeating him. When he throws a blue energy ball at you, hit it with your sword to counter
it. You probably knew this already, but you need to do its more difficult at
Ganon’s Castle. Phantom Ganon also has two other attacks now. One is a form of red
energy, when he’s done charging it up, it will split into multiple red energy
balls and try to hurt you. Do avoid this, do a regular spin attack to deflect all (or
most) of the energy balls. They’ll fly back and hit Phantom Ganon. The last
technique is when Phantom Ganon splits into multiple Phantom Ganons and surrounds you.
Don’t let him surround you, when they attack with their swords, they’ll
fuse back into one. This works but if you do it wrong, you might lose a lot of life, when
the Phantom Ganons attack with their swords they immediately turn back into one. If you
are close enough, the one Phantom Ganon can be hit with your sword and he will die. But if
you are too close when his clones attack, you’ll lose a lot of life.
Going Through the Puzzle-
After you defeat a Phantom Ganon, his sword will be left. DON’T TOUCH IT. As you
will notice, there are three doors in each room with a Phantom Ganon. Wherever the sword
hilt points to is where you should go. The sword hilt is the part of the sword that you
hold.
Getting the Light Arrows
One room has only one door and it should be locked by these vine-like bars. After
defeating this Phantom Ganon, a treasure chest will appear in the center of the room.
Inside the treasure chest are the Light Arrows. Take them and the door will open. 
The Last Phantom Ganon-
The only way to kill the last Phantom Ganon is with the Light Arrows. Shoot him with one
and he’ll die. There should be some kind of strange marking on the brick wall in
that room. Pick up Phantom Ganon’s sword and smash it, the exit should be there.