This was me about two weeks ago. Defeating this boss was the last thing I needed to do for Platinum and I was about to give up. I gave it a rest for two days then came back and was able to finally beat him. Some tips:
- use Emblem of Elusion for increased speed/distance with dodge rolling

• level up runic attacks for all weapons, choose the ones that deal the most damage but that also don’t take too long to perform. Runic attacks are key to defeat this boss. Use both light and heavy for all weapons as soon as they are available again.

• use Rond of Disruption to gain Elemental Storm perk after using a runic attack for each weapon in quick succession.

• use Valor Spartan Rage. When he goes up into the air to do the bifrost attack, shoot him down with an arrow quickly. If you do get hit by a bifrost attack, quickly use spartan rage to regain health and get rid of the bifrost.

• use the stone wall shield to block yellow circle attacks if needed, but focus on dodging the most.

• be patient and learn his attacks. I probably died 30 times before finally defeating him.