There is a difference in showing the ability to use the Force and defeating the First Order's best enforcer in your first lightsaber battle (first time even holding a lightsaber actually). I don't care if Kylo was hurt -- I thought the scene made him look too weak and her look too strong.
The Mary Sue critiques are valid, IMO. It becomes even more absurd when you take a step back and compare her progression to Luke's.
How long did Luke train? There was the battle of Hoth, Luke traveled to Dagobbah. The Mil. Falcon was chased, hid in the asterroid field, floated away with the garbage, traveled to Bespin, had dinner with Vadar, and Luke was there within a day or two. If you stretch that timeline to its breaking point, he trained a couple of days and held his own against the Sith Lord/Hero of the CLone Wars.
The point is that Luke Skywalker was extremely strong with the force, as Rey obviously is as well. They are super-Jedi.
Not sure how much of an argument, "I don't care is." When someone is shot through the side by a weapon they spend the whole movie bragging about, and then fights like its nothing, that's equally implausible to what you are suggesting.
Also Kylo Ren was also shown as being visibly shaken by rey's inate powers--she freaked him out. He was scared in the interrogation room by her, and he was shaken up while he was searching for her, claiming she was growing stronger. They spent the movie showing that KR loses his cool and his focus in certain situations. He was completely internally-conflicted whihc made him WEAK. That is why Snoke wanted him to kill Solo. He had to get that ruthless focus.
They also went out of their way to show that Rey was competent to defending herself with a staff, so she has some muscle development/memory in that realm. It's not sword fighting, but she wasn't a waitress or school teacher.
And personally, I just can't stand hearing the term "Mary Sue" over and over. We need a new term. It's like those message board posts that get into Nancies and Pollys. Please no more.