I seriously doubt Serwa will be dead -- unless TUC was the final installment of the series, then killing her off now feels like a huge waste of the character, which is almost certainly why RSB left it ambiguous. I also think Serwa still being alive would be interesting simply because I think she's the only one who WOULDN'T instantly believe that Kellhus is actually the No-God. She was clearly the one who knew the most of her father's true plans for the Great Ordeal (up to and including the Proyas betrayal), so I could see a lot of dramatic tension being created in whatever sort of counter-force is assembled based purely on that.
However, discounting Serwa, I do think Achamian is probably the most powerful sorceror in Earwa.
Honestly I think this is the point wherein Achamian's characterization will deviate the most from its status quo in the series thus far. He is now, really for the first time, completely "in his element", even if he didn't realize it to be such. The No-God walks -- Seswatha won't stand idle.