At first, the hardest boss battle I had was the Yuga/Ganon final battle from A Link Between Worlds. I had no idea about what to do, and I kept dying.
Then I learnt that you were supposed to use your sword to deflect back the orbs which are hurled at you, and then after that, you are given the Bow of Light (painting use only)
You may say that it became easy for me afterwards, but before that, it was too difficult for me