Meh, I see your argument. While I believe that OP was kinda bs, the final battle was probably only win-able through the fact that they had all been nearly killed in the battles in their respective throne rooms and two members already being dead and returning as spirits and, in my observation, were doing so through drawing on the power of the Dread Palace and Oricon to sustain their forms during combat. I can definitely understand your point, and the flaws in pretty much any made for the Dread Masters, I just can't see her taking on two of the actually impressive members (combat wise) alone.
True, idk why but I interpreted it as they amp each-other so collectively they're stronger together, but I forgot that it's the way you stated. None the less, I personally don't see Shan beating either of them 1v1, but that's just me. I'ma go to sleep now, I'll debate this in the morning.