Zelda has sealed Calamity Ganon for 100 years, and now Link must defeat his ancient foe once and for all.
If you have not completed all four Divine Beast dungeons, you will have to battle the bosses from each one that you have not yet defeated: Waterblight Ganon, Thunderblight Ganon, Windblight Ganon, Fireblight Ganon.
Calamity Ganon: First Phase
You can take a picture of Calamity Ganon for the Hyrule Compendium if you wish.
The key to winning this battle is performing Perfect Dodges and Perfect Guards. As you may already to know, to perform a Perfect Dodge, target the enemy and then do a backflip or sidehop just before the enemy's attack hits you. If timed correctly, time will slow down, and you can tap Y repeatedly to perform a Flurry Rush. To do a Perfect Guard, hold ZL to raise your shield, then press A just before the enemy's attack hits you. If timed correctly, the enemy's attack will bounce back. This can be used to shoot projectiles back at the enemy, damaging him.
Throughout the battle, keep your distance from Calamity Ganon, because if you are too close to him, he will periodically generate an energy blast that will hurt you.
You should keep your shield raised for the entire battle if possible. If you run out of shields, consider opening your map and pressing X to escape so you can gather some shields.
Like Fireblight Ganon, Calamity Ganon will swing his flaming blade, which you can Perfect Dodge to use a Flurry Attack. The blade shoots flames outward, so be careful to avoid the flames. These flames will also create an updraft around Ganon, so you can ride the updraft by jumping and using the Paraglider, and then shoot an arrow in slow motion at Ganon's eyes to stun him, although this is risky because the updraft might be very close to Ganon. While he is stunned, you can attack him with your melee weapon.
Also like Fireblight Ganon, he can throw a glowing ball of fire at you. You can stop it by shooting an Ice Arrow at it, or you can run away. It creates a large explosion, so be sure to run far away to avoid the blast radius.
Like Waterblight Ganon, he can create blocks of ice. Use Cryonis to shatter these before they hit you, or just run away from them.
Also like Waterblight Ganon, he has blue blades that he shoot at you or swing at you (much more quickly than the orange blade), as well as smaller blue weapons that he can attack with at close range, so keep your shield raised. Try to Perfect Dodge these attacks if you can.
Like Windblight Ganon, he can create tornadoes by swinging his blade, and the tornadoes can't be stopped or destroyed, so you will have to run away from them. He can also shoot a blast of air directly at you, which you should also run or jump away from.
Like Thunderblight Ganon, he can drop pillars into the ground and shock them, so be sure to run from those. It may be possible to use Magnesis on them after he stops dropping more pillars.
Ganon can also climb the walls, and you might have time to shoot him in the eyes before he attacks from the wall. When he jumps off of the wall, keep your distance so you can avoid the blue blast of energy that he emits when he lands.
He can also shoot energy blasts at you, as well as a stronger energy blast that he shoots after aiming a red laser at you. You can deflect these with Perfect Guards, but the stronger energy blast will destroy your shield if you do not deflect it in time, so you should run away from the stronger blast unless you are very confident in your timing.
Urbosa's Fury will stun Calamity Ganon, giving you an opportunity to attack repeatedly with a melee weapon.
Calamity Ganon: Second Phase
After you have reduced Calamity Ganon's health to half of what it was at the start of the battle, he will create an orange force field around himself. He is invulnerable while his force field is up. The shield only disappears just before he attacks, so it is necessary to do Perfect Dodges and Perfect Guards to be able to hit him when he is vulnerable. His attack patterns are the same as before.
Dark Beast Ganon
After you defeat Calamity Ganon, he returns in the form of Dark Beast Ganon. (Take a picture for the Hyrule Compendium if you wish.) The battle continues outside and you will get on a horse (the first one that you registered at a stable, or a random horse if you never registered one). Your horse will ride to the Bow of Light, and you will automatically take it when you touch it. Be sure to equip it or you won't be able to do any damage.
Avoid being in front of Ganon's face, because he will spit powerful beams of energy in front of him. Ride around Ganon's sides while you wait for glowing symbols to appear on his body. Shoot the glowing symbols with the Bow of Light. There are three symbols on each side of Ganon's body. The Bow of Light has infinite arrows, so feel free to shoot rapidly. The symbols will disappear after a while if you don't hit them in time. If this happens, just ride around until the symbols reappear.
After you shoot the six symbols on Ganon's sides, wait for a symbol to appear on his chest, then ride underneath him and shoot it.
After this, ride out from under Ganon and watch his face, but don't stay directly in front of him. Eventually a Glowing Eyeball will appear on Ganon's face. However, you can't damage it from horseback. You have to wait for Ganon to spit a beam of energy that lights the grass on fire, then dismount your horse and float up on the updraft, then shoot the Glowing Eye while airborne. Be sure to aim at the large glowing eye and not at any of the smaller ones surrounding it.
Congratulations! Be sure to watch the credits (or skip them by pressing X and then +), because your game will be saved afterward, and that save file will have a star on it. If you load that saved game and open the map, you will see your completion rate in the corner. Each of the Divine Beast dungeons counts towards the completion rate, and defeating Calamity Ganon counts as well. You also need to find all of the Korok Seeds, complete all of the shrines, and find every named location on the world map.