So all this time, Kinemon's purpose in learning how to cut a flame was for this moment, eh? But they won't win with just that. Kaido has thick skin. Their slash can only graze him.
That would really be deviating from the standard shounen flow, but I also hope they choose to go down that route. It would be so much more meaningful if the samurai can get their revenge by their own hand. Big Mom is an equal threat; Luffy can take care of her.