Warning: Spoiler! (Click to show)
I would like to put forth that Rey besting a Ren Kylo who had been 1) shot by a crossbow that usually blows people up completely 2) just killed his own father and was not in the best emotional state, maybe, 3) was already inexplicably wounded by Finn, who has about as much experience with a lightsaber as I do, is not that unbelievable in the Star Wars Universe.
What is pretty unbelievable is that Finn didn't just get chopped down immediately by Ren, but that, all things considered, did pretty well. After all, Finn, using a lightsaber, had, just a day or so ago, had his ass kicked by random lightsaber safe tonfa wielding stormtrooper.
Current Day Mark Hamill reminds me of a dude I knew in LA who is a journeyman guitarist for C-level bands.