Yeah the NG+ Lost Sinner adds are basically the same pyro guys that are part of the Umbral convenant run. Their pyro spells do lots of stun and they have enough hit points to avoid getting single comboed down, like the adds on Flexile. Sinner himself is kind of a dick with his guard penetrating pierce attack and huge jump backs. One thing though is that those pyro guys do tend to go down fast to lightning bolts, which tend to out range them, but getting enough space from Sinner to pull one off is the trick. If you insist on soloing it, you can try gimping the fight with poison daggers. They are fast launch, use almost no stamina, and everything in that fight is susceptible to poison. I got past it on NG+ with my caster guy by vortex + immunity hex cheesing to clear the adds. I have heard that you can reflect most of the adds bullshit with the looking glass shield, too, but its supposed to be insanely hard to pull off. No fucking way am I trying that guy solo with my mele dude in NG+.
On a side note, is it just me or is Adaptability mostly useless? I dropped a bunch of points in it on my mele guy, but it seems to make no noticeable difference. When wearing the same armor, there is no difference between my caster guy and mele guy, despite nearly 20 points of adaptability separating them. Anyone else have any input on this?