I've beaten the Gaping Dragon and Quelaag with a level 15 sorcerer. I summoned Knight Lautrec and Solaire for the dragon (both were pretty useless), and Maneater Mildred for Quelaag. In the latter fight I didn't even get hit, but the Gaping Dragon was quite a challenge. I didn't get to cut off his tail, unfortunately.

Keeping my level low, all my souls pretty much go into spells (both sorcery and pyromancy) and (upgrading) equipment. The damage outputs of the various Soul Arrows aren't impressive at all since my INT is only 18, but the Dragon Ring that boosts sorcery helps. That's 20.000 souls well spent! My main melee weapon is a Rapier +5, which clearly doesn't cut it in Blighttown. Killing the mosquito's is a real pain with the lack of a slashing motion.

I might dash through the Catacombs before entering Sen's Fortress to upgrade it to a Fire Rapier, or I'll make it a magic one and start boosting my INT. I'll also try to beat the Hydra in Darkroot Basin so I can find Dusk of Oolacile, an NPC I didn't even meet on my first playthrough. Funfunfun