Calamity Ganon is the mastermind behind all the destruction in Hyrule. The enemy awaits in Hyrule Castle's Sanctum and you should really treat this fight seriously. First of all, Bomb/Ancient Arrows will be very, very useful. You could also use an x3 bow, in order to get that damage really high. You can reach the enemy without releasing the Divine Beasts, but that's not advisable. If you've skipped some dungeons and haven't defeated their bosses, they will appear before the fight with the Calamity Ganon. If you did, on the other hand, they will fire a powerful beam that will devour 50% of Calamity Ganon's HP! The fight consists of two stages:
The enemy uses attacks similar to the previous, elemental incarnations of Ganon. It will attack with fireballs, use strong melee attacks as well as climb walls and charge from them, creating strong earthquakes. Apart from that, it also has guardian weapons combined with a powerful laser that can take many hearts. Fortunately, you can deflect the attacks straight into the enemy, which will stun him. When you start the fight and the enemy does have only 50% of its HP, this stage shouldn't take long. You should get some Ancient/Bomb Arrows and continuously fire at the enemy, only stopping when you have to deflect the laser attacks. Don't waste time on melee combat and keep your distance.
When the enemy is left with around 25% HP, stage two will begin. The enemy will create a shield and then consume it, which will render him immune to all attacks. In order to take the shield down, you will have to deflect the laser into the enemy, which will stun him, allowing you to deal damage for a short period of time. Just as before - use arrows to do that. The attacks will not change, and so should your strategy: wait for the laser, deflect it and fire at the enemy. Eventually, the enemy will yield and you will be victorious.
Congratulations! You've just finished... But not so quick! The enemy will return in an altered form, and you will have to fight one more time.