NobodyReally
Superstar
She didn't "master" the force.
It channeled itself through her briefly, as for that fight Kylie was playing with her for damn near all of it and dude was half dead, would have been totally different if he was 100%. Plus he hasn't even completed his training which Snoke mentioned.
The trailer is a teaser, it's not suppose to give away much if at all. And I like things that way, as good as the Spider-Man and Thor trailers were they gave away quite a bit but that's also because they get released earlier than TLJ.
Judging the teaser trailer like a full trailer is inane, it served its purpose.
Your post doesn't make any sense. So Kylo Ren is on death's door but he can defeat a uninjured grown man like Finn but against Rey, he needs his full strength? And she may not have mastered the force but she figured out how to do Jedi mind tricks and fight with a light saber when we watched Luke train for two movies to learn the same. It was sloppy writing and you trying to rationalize it only highlights that even more. Think of how powerful it would have been if Rey had lost or hell even if she just barely came up even. The whole teaser about her being trained would be 10 times more exciting.
And they don't spend millions making teasers for them not to be judged. The purpose of a teaser is to excite. I'm not excited, that's my opinion.