I think Phantom Ganon was the hardest boss for me. Not because of difficulty but because of the way you had to fight him. First off when I fought him the first time I ran out of arrows and had to die on perpose, then I couldn't even find any arrows inside the dungeon, so I had to go outside to get some from wolfos I think. And the other reason is because when I beat him on his horse and had to deflect the bolts the camera view was akward, the railings would get in the way and it would be hard to time the attacks. If I were to go closer I would be too close to Ganon and would end up getting hurt from his bolts because I wasn't far enough away to react and slice the bolt.