Like all boss fights in Breath of the Wild, Fireblight Ganon splits his attacks into two phases — one when his health is above 50 percent, another when it’s below.
Fireblight Ganon Phase 1
The trick to defeating Fireblight Ganon is to get close. Underneath, he doesn’t have much of an attack or defense to mount.
If you’re not underneath him, and he performs his sweeping or other sword attacks, block or try to get away from him when he does. If he’s flinging fireballs, run behind pillars for cover.
With the Master Sword, it doesn’t take much to knock his health down below 50 percent and begin the second phase.
Fireblight Ganon Phase 2
Fireblight Ganon’s second phase is mostly about him hanging out above you, hurling a single, large fireball.
If you followed along with our guide above, you should have a good stock of ice arrows at the ready (from treasure chests within Divine Beast Vah Rudania). One shot with them will destroy the fireball. If you don’t have any ice arrows, just hide behind large objects to avoid getting hit.
Don’t try attacking while he’s surrounded by a yellow orb. Nothing will get past him. Except …
To create his fireball, Fireblight Ganon sucks in all that’s around him. This is the key to breaking the attack. Toss a remote bomb his way, and detonate it. That’ll break his shield and make him vulnerable for attack.
Again, with the Master Sword, he goes down quickly. He still has some of his previous attacks (augmented by a flame on the ground), and the safest bet is to avoid them, block them, and at every opportunity make your way underneath him, where he’s most vulnerable, and you’re best able to attack him.
Hurt him enough, and he’ll fall to the ground. This is your opportunity to unleash every attack you have.
Completing the Divine Beast Vah Rudania main quest
Defeat Fireblight Ganon, and you’ll get a heart container. You’ll also receive Daruk’s Protection, which will "automatically protect you from all manner of attacks as long as you’re holding ZL."
After a cutscene, you’ll be warped to Goron City. You must speak to Bludo to complete the Divine Beast Vah Rudania quest. On your way, you’ll run into Yunobo for no particularly good reason. Just walk straight ahead, and talk to the chief.
After your conversation, go into Bludo’s house (behind him), and open a treasure chest there for the Boulder Breaker, a powerful two-handed weapon.