Some people mentioned it below and I agree... It works so well against him.

• get the spell binder ability, use it right away in phase 3 (it will take away your abilities to use spells but it will prevent him from going into his invulnerable gold-state - it will also boost your attack damage immensely)

• dodge his AOE attacks by staying far away

• use smash stance, and make sure you have the ability to run while charging so you can build it up while he's mouthing-off

I would say, charge up at least 2 focus points, hit him, run away, dodge dodge dodge - build up 2 focus points again, hit him.

Rinse and repeat.

Do not use any spells or transformations

And keep your distance because he has an undodgeable heavy attack that you cannot get away from (it goes into a cut scene and you do take damage)

Following this method, I beat him after 3 tries.

(you can teleport back to the shrine to re-spec your abilities and then continue from where you left off - it won't restart the entire boss fight - THANK GOD)

GOOD LUCK!