I really don't see the issue with Rey's aptitude with the force throughout the film, or her managing to hold off Ren and cut him down to size, whether Rey has been force mind-wiped or not.
Ren starts off the movie appearing to be an incredibly capable and competent force user. He gives off an aura of confidence in his initial appearance, and the introduction of Force Stasis in the cinematic universe through him creates a striking impression of power. Similarly, his use of the force in interrogating Poe and breaking him, again, gives off the impression that he's a competent individual with a strong understanding of the force. His ability to sense disturbances in the force re-emphasise his connection to it, and his first outburst makes him seem to be a frightening individual with a huge anger that results in a strong grasp of the Dark Side.
These initial impressions though, are gradually dispelled throughout the movie. Rey uses the force throughout the movie, but does so in a more subtle way than we usually see in the Star Wars movies. She uses the force in her knowledge of what to do, and has her abilities greatly enhanced beyond normal means. She has great reflexes and develops an 'intuition' as to what to do, an intuition and understanding that's gradually revealed to be an ability with the force. She doesn't understand what or know that she's using the force, she doesn't even (apparently) believe in the force or the stories of what have happened in the decades prior, and has lived a tough life on Jakku where she had to hone her natural survival skills and instincts to survive, making do with what she has in order to survive and having a somewhat challenging path, allowing her to get a grasp of variety of adaptable skills which become further enhanced by her very strong connection with the force. The journey she embarks upon with Finn and BB8 result in her becoming more aware of the force, and the reality of it, and the experience in Maz's allows her to become clearly aware of her strong connection with the force.
It's around this time, that Ren's own incompetence begins to become clear. While he has a connection to the force, he doesn't have the same intuition with it that Rey does, and clearly doesn't have as good of a grasp as we thought he did, as we reflect upon how he doesn't realise that he should have stopped Finn during their initial encounter, and he doesn't learn from the force that he should still capture BB8. Our biggest insight comes from the interrogation with Rey. Without training, Rey is able to resist him. Without training, Rey is able to push back into his mind, and learn of his strong insecurity that he isn't good enough, and isn't nearly as strong as Darth Vader despite his desire that he should be. She manages to goad him into taking off his mask, and he visits Snope desperate, afraid, and completely uncertain what to do. This fear that he's not as strong as Vader can only possibly be enhanced after Rey knocks him completely. After this moment, his actions take a completely different light. He has never, it seems, been challenged by anybody competent in the force before, or with much of a connection. Everybody we had seen him do battle with prior had been somebody without a connection to the force, he'd never been challenged, or faced a challenge.
Rey manages to listen to the force, and allow it to guide her, not with a knowledge of what's happening, but knowing that it feels right, and accepting it to escape, but Ren doesn't, Ren fights off his own feelings at every point, and is completely dominated by them. Whereas the first outburst appeared initially like a great anger that was fueling a grasp of the dark side, the second completely undermines that; he is completely unable to control his emotions, he's being torn apart by his insecurity, every failure crushes him and confirms that he isn't as strong as Vader and that he is actually weaker, he doesn't really know what to do and cannot contain his rage to strengthen his power but just lashes out childishly. We begin to see, from here, how his strength with the force is not as strong as we were lead to believe. While yes he senses Han on the planet, he can't use the force to sense that Han is behind him or nearby as he is is on the walkway, he continues to be tormented by the light side, and resist the pull towards it. He waivers continuously and has no idea what to really do. He doesn't listen to his instincts and isn't guided by the force, he acts how he wants to be, desperately trying to live up to Vader and desperate to be as strong as him, but clearly isn't. Snoke can sense this inner turmoil too, and uses it try and goad him into killing Han. He hopes that it'll settle his hesitation and inner turmoil, and allow him to become adept with the dark side and fuel his power, but it only further emotionally it cripples him in the moment even though he does manage to ascend Vader and murder family, but is then completely taken off-guard by Chewie and can't stop the blaster from hitting him.
He confronts Finn and Rey. He isn't overpowered by either, they're holding him back, and fleeing, more or less until Rey finally accepts the force and follows its guidance in the fight, and swiftly overpowers him. He has been trained in the force, sure, but he is desperate to live up to Vader. His emotional state is all over the place following the death of his father. He is desperately afraid of facing Rey, a character who has only confirmed his inadequacy up to this point. He has a powerful, legendary lineage and is afraid that he won't be able to live up to it, and has a huge amount resting on his shoulders. In his wounded state, he needs to prove not to Snoke, but to himself, that he is able to live up to Darth Vader's power and take out Rey, yet as soon as she accepts the force and takes the lightsaber, he is again confronted with the knowledge that he isn't the most powerful person in the galaxy. There's no doubt he's somebody powerful, if he was the person involved in destroying Luke's Academy he obviously managed to prove his competence to many of those in training [but again, none of them presumably had the same lineage that he had, and the same natural affinity with the force], his relation to Anakin, his position in the Knights of Ren [but it goes back to the same thing, he has a natural affinity with the force, how many of them can say the same], and his training from Snoke), but he clearly is not as powerful (potentially, anyway) as Rey is, doesn't have control over himself (in contrast to Rey who at that point has accepted the force, wishes to be guided by it, and is empowered by the dark side after the death of Han and the injury of one of her only friends, Finn), and really is not as gifted with the force as he wants, and needs, to be, leading to his huge insecurities which further crippled him. He was presumably forced into training, and told what he had to live up to, but just simply is not able for this, and it crushes him and makes him desperate in his quest for more power, power which he feels he can only obtain from a complete acceptance and knowledge of the dark side even if this isn't what he feels tugging him along and guiding him. I actually think it makes Ren one of the most compelling aspects of the new movie, and a rather unique character in the Star Wars cinematic universe/