Okay, this is mostly shameless bragging but I will now say that Orc Rogue + CYDSTEPP is way overpowered.
So that's the two nastiest bosses in the game thus far taken down without preparations - and I'll also note that I used very different tools each time. Against Namtar it was Mystera (extra mp and lots of piety) -> Tikki Tooki (Tikki Edge, extra dodge, lots of gold) -> Mystera (to avoid getting punished for using death protection) with lots of items bought; against the Avatar I went Glowing Guardian (Humility + Absolution for level gains and piety, then Enlightenment) -> Dracul (sacrificing all of my HP for extra max MP, and taking a level of lifesteal solely for piety to compensate for his punishing death protection) with no items bought. I'm not sure how this strategy should be nerfed, but I am sure that it should be nerfed.