NG+ four kings is harder than O&S solo, but its pretty close.
I don't mind the tag teaming as much as I did in prior games when I was more snobbish about soloing stuff. In DS1, at least, a fair number of the bosses were not significantly easier solo than with help like they are in DS2. And some of the DS2 fights are clearly made with at least one helper in mind (Medusa bitch, Persuer 1.0, Smelter, and Sinner 2.0) for the fight, usually with an NPC phantom as an option. When I did my second run through, I thought about trying to solo everything with the pure two handed character, but I really did not especially feel like spending hours on Smelter, so I said fuck it.
On the flip side, I really enjoyed helping people power farm zones and clear bosses. It was genuinely fun, if your builds differed, and good soul income.