Regarding the Ren/Rey discussion, I've seen several people complain that she could match him already. As I see it:
For starters, Ren was injured. He took a pretty bad hit from Chewie and you could see he was bleeding all over the place, which is arguably why he didn't beat down Finn in 2 seconds, but Finn managed to get a few swings before being defeated. But my main thought in all of this was, Ren is not a full on Sith like Vader was in A New Hope. He's an apprentice of the dark side who was on his way to becoming a Jedi and then turned, but if you stop and think for a second, a lot of the criticism about Ren being defeated by Rey comes from assuming that he's a powerful Sith that is unmatched in a lightsaber duel. What we know is that Luke was training a group of Jedi, who turned on him, and Kylo Ren was the main guy in this group. But how far into training was he?
If Luke was training new Jedi, it's not crazy to assume that he would use a lot of the same teachings that he got himself from Yoda, and as we all know, Yoda never trained Luke with a saber. To Yoda, becoming a Jedi and learning about the force was a whole thing on its own, and that the saber was just a tool. He probably had basic training, but then we get to the fact that Luke is described as the LAST Jedi, and he vanished after the Knights of Ren turned on him. So over the last 15 (?) years since Luke has vanished, Kylo Ren has basically been the Vader of the First Order, going in missions, storming into villages and flashing his lightsaber, but before he ended up in the duel with Finn and Rey, it's not far fetched to assume that Kylo Ren might not have been in a lightsaber duel since turning on Luke all those years ago, because Luke was the last Jedi and it's not like the galaxy was filled with Sith lords which he traveled around fighting either.
That leads into my final thought which is underestimating his opponent. Rey was not a worthy adversary in Ren's eyes, and so when they engaged in battle, he was taken off guard by her power. She may not have fought with a lightsaber before, but she was handling her staff really well, so she definitely had the moves learned fairly well. Also, with Kylo Ren mainly going around the galaxy and cutting down innocent civilians in villages they raided, it's not like he met any real opponent. When he stood against Rey, she was also strong with the force and he didn't expect it. So with Ren, this could very well have been his first "real" lightsaber duel (because of the lack of Jedi and Sith, and the lightsaber being a very rare weapon) since his training at least, and if you compare that to someone like Vader who was in the Clone Wars, who fought big opponents like Dooku and Obi-Wan, he definitely had the training and lightsaber skill down, and was therefor a much harder opponent for someone like Luke, who had zero experience in Empire Strikes Back.