I really don't want to rehash the whole Rey v Kylo fight with a wall of text so I'll try to keep it short.
-Kylo was injured by a weapon shown multiple times through visual cues to be strong enough to send an ordinary human flying.
-Kylo just killed his dad and was unfocused.
-Kylo was visibly just toying with Finn who he wanted to make suffer for his defection until Finn got a lucky shot in. Not even 5 seconds pass from that point before Kylo ends the fight by dropping Finn.
-Rey spends almost the entirety of the fight either on the defensive or straight up running away from a heavily injured Kylo until she gets forced to the cliff's edge (it drives me insane that no one seems to acknowledge this)
-while on the cliff face Kylo is no longer concerned with hurting or killing Rey, he instead tries to get her on his side. I mean the whole time Rey has her eyes closed Kylo could have killed her at any moment.
-Rey focusing in on the force really only seemed to give her a little energy boost to break from Kylo and catch him off guard.
1) Kylo was merely grazed in the love handles by Chewie's bowcaster. None of his organs or major muscles were hit. It was likely only painful, and it's well known that adrenaline disguises pain. He certainly doesn't appear to be "heavily impaired" once the fight starts. I think that his injury is getting exaggerated to try to explain Rey's performance.
2) It can be argued that killing his dad required a lot of focus, but, regardless, it can be argued that Rey was also unfocused from watching an almost father figure be murdered in front of her and seeing her friend, Finn, perhaps dead in the snow.
3) I don't see how Kylo toying with Finn explains Rey defeating him. If anything, it suggests that Kylo was
not too injured to toy with someone not on his skill level. After all, if his injury and lack of concentration excuse him losing to Rey, shouldn't Finn have done a lot better against him, himself?
4) Rey spending the majority of the fight on the defensive doesn't matter when the point is that she won in the end, not that she dominated him.
5) Similarly, I don't see that it matters that Kylo had the advantage at one point and didn't press it. If the implication is that he could've won if he wanted to, why didn't he do something about it when Rey was landing blows and forcing him to the ground?
6) Her Force use isn't the only element that makes the fight unbelievable. It's also her mastery of a lightsaber just days after picking one up for the first time.
What it comes down to is this: are we really to believe that someone who mastered the Force and lightsaber combat was so impaired by a relatively non-serious injury and a lack of focus that he could lose to someone who had never used a lightsaber before and didn't even realize that she could use the Force until
the middle of the fight? So much for the once important franchise convention that Jedi and Sith masters are so powerful that they can only be stopped by another of their level. Apparently, now, all that it takes is for them to be in a little pain and have conflicted thoughts and an amateur can vanquish them. No wonder the Jedi are dying.
I mean people are free to not like TFA but I always feel compelled to fight back against the ongoing narrative that it's unbelievable that Rey beat a trained force baddie in combat. If anything the movie is overly blunt in generating the context clues and conditions that allowed Rey to get the upper hand. It absolutely blows my mind that people stay so surface level on that aspect of the film that such simple context is so widely missed.
It wouldn't blow your mind if you tried a little harder to understand the other side's argument. No one is "missing" the context that you feel is so important. We just don't feel that it adds up to make the outcome believable. The fact that it does add up to you doesn't mean that others are looking only at the surface level and missing context.