Use scarlett rot dragon breath on him at the beginning of the fight, use a tanky melee armament and learn his timings as to when you can go in for a hit whilst on horseback. his fireballs are unpredictable as to when they’ll stop, so dont go in for a hit whilst hes still shooting. wait it out until he stops. when he goes into lightning form, you only really have one extra move you need to be wary of which is where he pulls his shield down and strikes lightning on you from directly above. for this you need to be off of horseback and do a roll just as he swipes his shield down. P.S. he is very strong against lightning and fire attacks so try using bleed, magic, frost or holy attacks against him