Anyone who says Luke beating Vader in a duel is more absurd than Rey besting Kylo wasn't paying much attention to Luke's training, or the subtleties of the duel really. In Empire, after Luke has had a considerable amount of training with Yoda (a couple of months or so), Vader is mostly toying with him so that he can keep him alive and deliver him to the Emperor. Had Vader really wanted to kill him, he could have at any time, as evidenced by how quickly he dispatches his hand and lightsaber once he gets annoyed.
By Return of the Jedi, Luke's abilities have grown. More time has passed, and he has continued his training on his own while he prepares to rescue Han, prior to going back to Degobah. This is clear early on as well, since he is far more confident (almost arrogant) when going to Jabba's palace. So, when he faces Vader again he is more prepared, and is able to at least fight him to a standstill. It isn't until he lets his anger flow through him that he can actually overcome Vader at all.
The point is, Luke did have a degree of training, and time to explore the Force, before being able to overcome his adversary. Rey doesn't seemingly have the same arch, although I'm willing to bet she has had training in the past, but was mind-wiped to not remember it or something. Her ability to so easily use the Force without training is kind of silly, but I suspect that we will learn that she did have some training prior to this. Also, training in the use of the Force is not the same as being able to use a lightsaber, but she may have had some of that too.
I have far more problems with Finn being able to hold his own with Kylo injured or not than anything that happens with Rey. Even injured, Kylo should have been able to dispatch with Finn without a problem. The fact that Finn hadn't even touched a lightsaber a few days earlier, but can now hold his own with someone who has trained extensively is kind of silly. I've trained with swords in martial arts, and when you don't know what you're doing, they can be dangerous to yourself, let alone others. Even injured, Kylo should have had no problem defeating a guy who has zero training, and barely knows which end is up on a lightsaber.
Put it his way. How close would a one-on-one basketball game be between a guy who plays every week in a local rec league vs. someone who has never touched a ball before? What if the person played college or semi-pro ball, let alone in the NBA?