My first playthrough was done without summoning help, and I remember being in your shoes on that boss, so much rage lol. Nowadays I usually have a +5 divine weapon at that point, or even a +14 normal weapon (New Londo isn't that hard at around SL 40) and know to invest early stats in Vit and end. When you come back on later playthroughs once you really know what you're doing, you'll stomp them every time.