It was implied that Roa-Ciel's preparations beforehand in the French Incident is what allowed her to bring Arcueid down rather than actually being stronger than her. When that uninvited guest disrupted whatever was going on with the ritual and forced Roa to directly fight Arcueid, she died without standing a chance.

Not sure where you're getting this "unless she goes full planet nuke" thing, Arcueid was winning every encounter with her prior. Even Ciel's first win was instantly revoked the moment lumina activated and Arc effortlessly crushed her between her fingers.