Hit its hands with arrows (you can also use the Hookshot/Longshot, but it's slower), use the Lens of Truth to see its eye, hit its eye with an arrow as it charges towards you, and when it's stunned, slash it with your sword. While the Lens of Truth isn't actually required to hit its eye, it makes it easier to see where it is.
the last fused shadow is in the temple under lake hylia iafter you beat the boss you will get it
you have to beat the fire temple.
Go through the Shadow Temple, he is the boss.
beat every story boss and vs devil doom you are super shadow beat every story boss and vs devil doom you are super shadow
After you beat the water temple.
you have to beat goht the boss
beat the boss
Go To Hurle Castle And Save Zelda
Beat every stage and boss to reach the final story, and beat it. Devil Doom is the final boss.
you beat the last boss
In the Shadow Temple from a chest that appears after defeating the mini boss Dead Hand.
It is somewhere in the where in the Wind Temple