It does not one-shot any enemy, but it does more damage on The Blight Bosses and the Final Boss.
And when I say the Master Sword is Unbreakable, I Mean that, it can run out of energy, and when it is out of energy you cannot use it for a few minutes, the energy Mechanic works exactly like the durability thing on other Weapons, but the master Sword comes back after "Breaking"
RE: So I beat Breath of the Wild.