Yeah, Rebecca is pretty good defense-wise... she was dodging all those gladiators at Coliseum... but I still think a top tier attack from Nami (Thunder bolt?) could KO her, even more when Rebecca doesn't seem to like fighting in the offensive, which could create the opportunity for Nami to strike her.
Oh Mirage tempo, I forgot about that. If Nami uses that as a distraction and hits Rebecca with a thunder bolt then she would probably win. But do you think Observation Haki would let Rebecca know where the real Nami is?
Yeah, but she used the Mirage-tempo already kind of far into the fight, no? Before that I recall Kalifa hitting her some times. Considering Rebecca uses a sword to fight, it's possible her strikes would be more fatal to Nami if she is striken, that is.
Rebecca wins due to her Kenbunshoku Haki allowing her to dodge and predict Nami's movements. A hit from the clima tact's lightning would KO her, given it was able to stun a Dragon at least as strong as the one G2 Luffy and Zoro struggled with, though the chance of it hitting is very low.