Ok while I still have not beat the game yet, I recently started a SL 1 game. Currently at Sen's. I gotta say Quelaag is really easy for me now. At first I kept dying but now I have beat her twice on first try with the level 1 using +5 reinforced club with lightning pine on it. Now I an pretty broken cause I had enough souls to get pyro up to +13 and then defeated gargoyles on church without taking any damage.
I can't honestly say that I have an easier time with any of the bosses now than I did before. At least not enough to say "Oh yeah, this boss kicked my ass, but now they're easy".
I never really had any really big issues with any of the bosses, and the ones that I did have some trouble with I still tend to have some trouble with even now. The troublesome bosses are troublesome for good reason, and even with practice they can still whoop on me pretty good even on a good day.
I sometimes feel as though life is a curse that has been placed on the living. I envy the unborn at times.
on new game plus plus the bosses seem to have better AI and some even seem to break out some new moves (not new but they use attacks more often). You really should beat the game, New Game Plus was more fun for me thanks to that increased damage and enemy HP boost.