He commented that he thought it wouldn't be doable on NG+. My feeling is that if he can't do it no one can, but I'm interested in what would be the hardest bosses. My thoughts:
Bosses that wouldn't be an issue - Taurus, Gargoyles, Quelaag (especially with summons, she's still weak to magic), CD, S+O (easy to stay away from and spam magic at, especially Smough), Pinwheel, BoC, Gaping Dragon
Bosses that might be a pain -
Capra - have to use vegeta's trick of hidden body and hyper mode, but it still requires flipping around perfectly in a very tiny space as to avoid aggro-ing the dogs
Nito - more for the skeletons, since I assume they would take you out in one hit
Centipede Demon - I did fine with my heater shield the first time, but he may be strong enough to break your guard the next time around.
Bosses that would be a huge pain -
Gwyn - Requires you to parry perfectly about 30 times. That or run around a pillar like I did and use 40+ great soul arrows
Seath - Super resistant to magic. Even with seance + bellowing ring I'm not sure if you'd have enough magic. Probably have to go two-handed against his tail and it's going to take a while.
Demon Firesage - Can't stay far away because the ax tracks perfectly. Hard to stay close because if you get hit by the explosion you're dead too.
4 Kings - the granddaddy of them all. On NG I had to use all 40 great soul arrows. Blocking the ranged attack isn't an issue with great magic shield, but in this case you're going to have 2-3 other kings on you at the same time while being at range. If you try the flip strategy you may get one-shot just by a hilt blow.