Fireblight Ganon is a boss in Breath of the Wild. It was created by Calamity Ganon to corrupt the Divine Beast Vah Rudania, eventually serving as boss of the dungeon when Link came to free it of Ganon's control 100 years later. If Link doesn't fight Fireblight Ganon inside of Vah Rudania, he will later encounter the boss in the Sanctum inside of Hyrule Castle.
During the first phase of Fireblight Ganon, it will wield a massive axe and will try to attack you. Its vertical slashes can be dodged by jumping sideways, whereas its horizontal slash can be dodged by jumping over them. Alternatively, you can avoid everything by keeping your distance. However, We think the best way to defeat it is to strategically dodge one of its attacks, allowing you to perform a Flurry Rush, and hit him multiple times with whatever is your most powerful weapon.
During this phase, Fireblight Ganon will occasionally do consecutive horizontal slashes, so be careful. It will also occasionally throw out a bunch of small fireballs in your direction and I would suggest quickly running away or blocking them if you have a powerful shield. After half of its health is depleted, it will transition to the second phase of the battle.
This second phase of Fireblight Ganon is actually somewhat easier. It will now float higher above and you will no longer be able to hit it with your sword. Fireblight Ganon will shoot fireballs out at you and you can either hide behind a pillar, or more effectively, just shoot them with an arrow to disintegrate them.
The key to defeating Fireblight Ganon is to use bombs. When it is charging up its fireball attack, it is sucking all nearby objects in the air. Toss a bomb in its direction and it’ll suck it up, much like an Octorok. Detonate the bomb and Fireblight Ganon will come crashing down. Run on over and deliver some sword slashes.
Fireblight Ganon’s final attacks will come in the form of a laser attack, much like a Guardian. We suggest running behind a pillar to avoid taking damage. While it’s flying around the room, you can now use any projectile, including arrows as well as just launching items at it. If you run out of items, you can still use bombs to deal damage. Just keep at it until Fireblight Ganon has been defeated.