Artorias is making me question my Sanity.

I've been fighting artorias all evening, and getting him to about 2-50% every time I fight him. But it takes 15 minutes per fight usually... So I wonder if there is a way to speed up my damage so at least my attempts take 3-5 minutes to get him that low.

I have 25 Vitality, 28 Endurance, 34 Strength, 20 Dexterity, and 15 Int

I'm using a Magic Flamberge +8, A Dragonslayer Greatbow +5, Bloodshield+5, and Pyromancy Flame +11