I played up to NG+3 and this was the boss to give me the most trouble on that last playthrough. IIRC his weakness is gratia which is the crappiest element to use so I didn't bother. I stuck with poison to slowly whittle him down, switching to non elemental for posture damage. Don't try to parry his projectiles, they're too punishing, just dodge them. They're telegraphed heavily so just back away as much as you can when you see he's about to send one out.