The thing that I always die to is his lightning bolt attack, due to its quick speed and difficulty to predict. With metal weapons on you, the lightning bolt can sometimes repel and hit him (for some reason), dealing big damage to him, but you also take more damage, which usually results in you dying due to the rain also increasing the damage it deals to you. I've gotten really close to defeating him in recent attempts, increasing my maximum HP through the use of BDs.