The fight differs depending on if you got 3 or 6 Ancient Arrows.
Video Tutorial by The Zelda Enthusiast:
6AA Fight: Start off by shooting all 6 of your Ancient Arrows directly into Calamity's face, then step closer to him so that way he does an attack, immediately run left to the Naboris engraving on the wall, pick up 10 Bomb Arrows then turn to face Calamity and start shooting them at him while walking left near the wall, after about 4 shots stop shooting to pick up the Royal Claymore and then shoot the rest of your shots off in his face. As soon as he starts charging his laser prepare to parry it, once you do switch to your Royal Guards Claymore and turn away from his face, start a spin attack (hit him at least 6-7 times then slam, throw the claymore in his face (if this doesn't end phase 1 just finish him with the boomerang.
Phase 2: Parry his laser, run up to his face with Edge of Duality equipped, hit his face twice, then start a spin attack and move toward the back of him (near the spear) after he sticks his spear in the ground to get up, slam, throw your Edge of Duality at him and then start another spin attack with your Royal Claymore (Calamity Stunlock), spin until he sticks his spear in the ground to get up then slam, repeat this until you have ended the fight.
