This boss battle is extremely hard, but there is a relatively simple strategy you can follow that will get you through it.
When you start the fight, obviously make sure that you are maxed out on health and light/shadow magic. In his first form, the boss will do a dashing attack, a ground slam with a shockwave effect that you have to dodge, or he will charge up his hammer and leap at you. Basically, the strategy is to run in and get three or four quick hits before dodging away. If you're daring enough, you can unleash a bigger combo for more damage, but it is risky to stay close since he can do the ground slam. Hit and dodge as much as possible, but use your light/shadow magic sparingly, because you will need it later on.
After you defeat the first form, the second form is basically the same. He will rush you, swipe at you, or do a ground pound. All I did was kite him around the arena, forcing him to dash at me. When he did, I would get a quick combo off and roll away. It's very tedious, but with patience you'll kill him. Use light and shadow magic here if you have it, to kill him quicker and heal yourself. After his health gets low, you must destroy the 8 pillars, and go through another annoying quick time sequence before he's finally dead. Good luck with this frustrating boss; dodging and patience are the key.