The gargoyles still give me a ton of trouble. Bar O&S, they're usually the boss to give me the most trouble. That's why I put them off for a while. I had a +10 weapon before I destroyed them.
To be honest though, most everyone is going to give up at O&S.... they took me 2 days of constant attempts to beat. Its just crazy. For an idea on how this run works, watch this video. (He's using a slightly different version of the run, I customized it for me)
Dark Souls - PTDE - Calamity Ring Challenge NG - Assassin's Run (Part 1) - YouTube
Now, this is a run mostly dedicated to those who find PvE too easy now. I've gone from new game about 3 times like you, and this run has truly tested me.
I set up this run to really prevent me from using the many crutches I fall back on when the going gets rough. It's a personal thing for me, you can alter it however you like.

IF you want a challenge run, but one thats still feasible without becoming a masochist, then try a SLv. 1 run. These don't limit you at all gear wise, and it's a ton of fun. The only key limitation is being level 1, and it isnt as restricting as you might think.
Watch this (hilarious) run for an idea of how one works.
Dark Souls: SL1 NG+7 - Part 1 - YouTube
This run was done on NG+7, so his run is the peak of challenge for this kindof thing.