Based on everything I've ever understood about Star Wars, it seems that Force users (even those with whom the Force is strong like Anakin and Luke) usually require significant training before being able to wield the Force even somewhat competently. Rey's ability to use the Jedi mind trick--even on a rank-and-file stormtrooper--without any training already seems to push the bounds of plausibility. Yet compared to her other Force exploits, this is seemingly one of her more mundane accomplishments.
Rey doesn't even really consider the possibility of being able to use the Force until Kylo Ren is already digging into her mind. Somehow, though, she is able to fight him off and even penetrate his own mind even though he is (apparently) trained in the Dark Side of the Force and she has barely even heard of the Force. She also decisively beats him in a lightsaber duel. How is this possible for someone completely untrained to defeat a trained Force practitioner? Was she really that strong? Is Kylo Ren really that weak?