Como um usuário da Força, não treinado, poderia ser tão competente em usar a Força?

Você parece estar esquecendo que o personagem em questão já era um lutador mais do que capaz.

Mais cedo no filme;

We see her fight off two men (aliens?) with her staff in the marketplace while Finn watches.

Isto demonstra claramente que o personagem em questão tem alguma experiência de luta, e eu imagino que a habilidade de lutar efetivamente com uma arma branca é muito similar a lutar com outra.

Antes dessa luta, também descobrimos que;

The Force is strong in Rey, and that thanks to Kylo trying to get information from her, he seems to have awoken her Force powers.

Vale a pena mencionar também:

Kylo Ren is injured during this battle after being both shot by Chewie and stabbed by Finn. He's also emotionally and mentally exhausted after killing his father, so he clearly isn't operating at his best.

E além disso, descobrimos logo após a luta em questão que:

Kylo Ren has not completed his training, with Supreme Leader Snoke ordering General Hux to find Kylo Ren and bring him back in order to complete his training.

Em resumo,

Rey, a capable fighter who is strong albeit untrained in the Force just about manages to hold her own against Kylo Ren, a wounded and distracted trainee.

Isso é pura especulação da minha parte, mas acho que

Rey has been trained in the force, by Luke, her father.

After Ben became Kylo Ren, Luke believed it was a mistake to have trained anyone in the force, so he made her forget who she was and left her on Jakku, lest she turn to the dark side as Ben had.

A evidência que sugere isso:

The light saber calls to her, not to Kylo Ren. It wants to pass from father to son to daughter, not father to son to nephew.

The visions Rey sees when she first touches the light saber are more likely flashes of memory coming back. Note in particular the little girl (and if I recall, she says something like "don't leave me"), who is very likely Rey.

The only plausible reason I can see for R2D2 reactivating himself at the end (other than "convenient plot device") is Rey's presence. Being a droid and not sensitive to the force, R2 must recognize her either from his own memory or some description by Luke. (This part of my theory has been debunked. See the link in the comments.)

As your question points out, she picked up force techniques very quickly and without formal training. I think we are seeing an amnesiac starting to remember old skills.

I could be reading into this, but it seems to me that Leia recognizes her.

Duas coisas que me ocorreram e que não estão nas outras respostas:

Kylo Ren is repeatedly shown being unable to control his rage. This is sort of part of being untrained as Dr R Dizzle mentioned - a trained Sith would use his rage with deadly force instead of obscene outbursts against inanimate objects - but it really showcases how rough Kylo is.

Além disso:

He fought Finn prior to fighting Rey, and this fight is much more surprising to me than the one with Rey. Finn may have been trained in melee combat - we see a storm trooper fighting with a lightsaber-like weapon outside Maz Kanata's bar, but as far as we know he has no meaningful connection to the force. Others noted that Kylo's arrogance might be getting to him in the fight as well, which would have helped Finn a bit. I think that if Kylo Ren was any manner of Jedi or Sith, he would have absolutely dominated Finn. That fight should have lasted only a few seconds. Even though Finn still lost, he put up a decent fight and was still alive at the end. This is an extension of the not-fully-trained aspect as well, but it makes it very apparent that Kylo has a lot of room for improvement.

Indo além do escopo deste filme. Precisamos nos lembrar de Anakin Skywalker que era muito habilidoso com a Força, mesmo quando ele não percebeu o que estava acontecendo. Anakin foi capaz de vencer o torneio pod racer, apesar de ele ser um garoto e enfrentar pilotos muito qualificados e muito implacáveis. Sua habilidade natural de ver o que vem a seguir é fenomenal, e assim é a habilidade dessa jovem de usar a Força naturalmente, mas de maneiras diferentes.

Embora eu não possa apresentar nenhum caso para o adepto da força que não seja

the force is strong with Rey and it has begun to be awoken,

Vou propor uma explicação para o porquê do duelo não ter acabado rapidamente.

Deixando de lado a falta de treinamento de Kylo Ren e a falta de controle sobre sua frustração, e mesmo desconsiderando as lesões ocorridas antes do duelo, temos que lembrar quem usa o sabre de luz :

O Jedi e o Sith .

Sabemos que Ren, Snoke e a Primeira Ordem estão tentando destruir os Jedi encontrando e matando Luke, que é o último Jedi conhecido. Nós também sabemos que o Sith trabalha sempre em pares.

Sendo esse o caso, apesar de Ren ter treinado abaixo

Luke, and subsequently Snoke,

não pode ter havido nenhuma vez que ele teve um verdadeiro duelo de sabres de luz , Jedi para Sith. Qualquer que fosse seu treinamento, provavelmente nunca havia um cenário em que ele estivesse duelando até a morte contra um verdadeiro inimigo.

Ele não estava preparado para um duelo real com outro usuário de força, mesmo aquele que está apenas começando a usar a força, e é por isso que a luta foi mais equilibrada do que o esperado.

Kylo Ren had already received a shot from Chewie's bowcaster (shown to be extremely powerful,) and had been injured fighting Finn as well. If you were fighting a swordsman who had not been fully trained in swordfighting who had recently been shot and stabbed, and you had a naturally high aptitude, like Rey does with the force, you might be able to fend him off yourself.

Mesmo que o filme em si tenha sido bom e tenha tido um grande crescimento para o rey, um duelo e também uma cena antes são muito estranhos. Mas vamos começar com o começo, vamos?

He is too old you say The too old I always got the feeling that it was just an excuse so that the one who was declined does not feel that bad about it. In addition to that though the older one is the more bonds one has formed already and Jedi shouldn't have bonds as those can be used to make them fall to the dark side.

A cena 1 que mencionei

The interrogation scene with Rey. Ren was able to get into her mind until he made an error and said something that made her will go strong. At this time she EASILY overpowered someone who has trained such things FOR YEARS. As we know from the prequels raw force potential says nothing (anakin lost to Dooku in Episode 2......easily) without any training. So there are 2 options for this part. Either Rey had a training and forgot about it (during her unknown childhood part before Jakuu) or she has a WAY HIGHER force potential than Ren. High enough that she is even untrained way more powerful than he is.

O duelo

Rey was on the defensive and he clearly toyed with her....only for one reason (and despite his terrible wounds). He wanted to train her in the dark side and make her fall. He had her at his mercy until she closed her eyes and did what Luke did during the battle of Yavin. She calmed her mind and let the force flow instead of blocking it out. Then she beat him fully. Here we can only speculate how she could beat someone who has way more training than her with the force and the lightsaber than her. Possible is as stated above that she has training that she doesn't remember OR that she is WAY more powerful than him (would also explain how the Great leader and Ren could feel her awakening). Additionally pointing into the WAY more powerful direction is the fact that kylo tried at the beginning of THEIR duel to telekinetically call the lightsaber........and it flew but not to him but to Rey.

Um pouco aqui adicionalmente (pontos pessoais):

In addition though is what I personally find confusing about the duel. The stormtrooper who seemingly is NOT force sensitive easily shot ren first and then attacked him. Even with his wounds Ren should have easily been able to vanquish him without even getting a scratch. Then Rey beats him easily as soon as she does not act like she wants but lets the force flow. So aside from the two possibilities above there could be another one that Ren is just not as powerful as he wants to be. That is countered though by his blaster freezing in the beginning of the film. So he IS powerful. And she has to be stronger as it seems. One thing we know for sure though is that Ren is NOT as powerful as Vader was (yet). As it is still his fear that he never becomes THAT powerful.

Bem, para ser franco, ninguém disse que ela não teve nenhum treinamento. Nós ainda não sabemos quem ela é exatamente, ou quem derrubou Rey em Jakuu para começar. Por tudo que sabemos, ela poderia ter sido uma das novas "filhotes" de Luke em sua nova escola jedi. Se subscrevermos a teoria de que as memórias dela foram apagadas, ela poderia estar inconscientemente agindo por instinto em seu treinamento na infância. Teria sido mínimo, mas o mesmo acontecia com Luke e olhava o que ele realizava. Sim, ela era apenas uma criança pequena, assim como Anekin ...

When Kylo Ren has Rey in the chair and is trying to extract the map from her, he is accidentally sharing knowledge with her on using the Force for extracting information from another. Later during the fight seen on the disintegrating planet we have a long pause while they're fighting and Rey closes her eyes; she is extracting knowledge (essentially Kylo Ren's training) from Kylo Ren, then she opens her eyes, the action resumes, and she fairly easily trounces him. She got her training in the Force directly from Kylo Ren's mind. Coupled with her prior experience in melee combat and it's plausible.

Há algumas coisas importantes a serem observadas:

Rey's flashback vision has Yoda saying "Its energy surrounds us and binds us" (a memory of Luke's, and a line that Yoda says in Empire Strikes Back just before saying "luminous beings are we... not this crude matter"), and Obi-Wan saying, "Rey... these are your first steps" (the latter words of which extend beyond the duration of the vision, at least from the perspective of the viewer).

A menos que estejamos interpretando isso como:

(A) a memory of Obi-Wan's force ghost speaking to Rey at some previous time (like when she was a toddler taking her actual first steps),

então devemos interpretá-lo como:

(B) Obi-Wan's force ghost speaking to Rey in the moment.

Interpretação (B) seria consistente com o fluxo temporal da visão, uma vez que:

The vision starts in the distant past with Yoda's quote from Ep.5 and the Cloud City hallway scene, and it ends in the future with a vision of Rey's fight vs. Kylo in the forest. At the start of the future vision is when Obi-Wan says, "Rey...". Therefore it seems consistent to think he is talking to her in the moment, because his words come at the junction of past and future.

Se for esse o caso, e ele está falando com ela no agora, isso pode muito bem significar que, talvez durante todo o filme, Rey tenha orientação do além, muito parecido com o que Luke Skywalker fez no final do Ep. 4 quando Ben conscientemente o guiou para fechar os olhos e destruir a Estrela da Morte, e no começo do Ep. 5 quando Ben apareceu para ele e disse-lhe para ir a Dagobah.

Observe o paralelo:

When Kylo has Rey pinned against the edge of the chasm and he says, "You need a teacher, I can show you the ways of the Force," Rey is reminded of Maz's words, "Close your eyes. Feel it. The light. It's always been there. It will guide you." She closes her eyes, then proceeds to do the near-impossible.

Isto é semelhante ao Luke fechar os olhos no final do Ep. 4., em seguida, procedendo a drenar a foto perfeita.

Então, sim, Rey tem alguma orientação. Se você quer acreditar que a própria Força a está guiando, ou que o fantasma de alguém está guiando-a através da Força, é a mesma coisa, porque para ter influência no mundo real através da Força, deve-se tornar um com a Força. Portanto, não há nenhuma distinção entre Rey sendo guiado pela Força em si, e Rey sendo guiado por aqueles que se tornaram um com a Força. (O significado de "um com" é especificamente que o eu e a Força se tornam uma coisa).

Como o fantasma de Qui-Gon diz a Yoda no Ep. 3:

Qui-Gon's voice: When I became one with the Force, I made a great discovery. With my training, you will be able to merge with the Force at will. Your physical self will fade away, but you will still retain your consciousness. You will become more powerful than any Sith. Yoda: Eternal consciousness. Qui-Gon's voice: The ability to defy oblivion can be achieved, but only for oneself. It was accomplished by a Shaman of the Whills. It is a state acquired through compassion, not greed. Yoda: ... to become one with the Force, and influence still have... a power greater than all, it is. Qui-Gon's voice: You will learn to let go of everything. No attachment, no thought of self. No physical self.

Mais tarde, Yoda conta a Obi-Wan sobre isso e se oferece para treiná-lo, e Obi-Wan fica surpreso.

Obi-Wan: Qui-Gon? But, how could he accomplish this? Yoda: The secret of the Ancient Order of the Whills, he studied. How to commune with him, I will teach you. Obi-Wan: I will be able to talk with him? Yoda: How to join the Force, he will train you. Your consciousness you will retain, when one with the Force. Even your physical self, perhaps.

Agora sabemos que Luke nunca terminou seu treinamento antes de Yoda se tornar um com a Força, e portanto Luke nunca teria tido a oportunidade de aprender o segredo da Ordem Antiga dos Whills. O fato de que Luke perdeu esta formação chave explica a seguinte linha do Ep. 7:

Han: The people who knew him the best think he went looking for the first Jedi temple.

Este templo quase certamente é o lar dos Whills. Certamente vemos Luke sentindo a compaixão necessária :

(From Ep. 7 script): IT IS LUKE SKYWALKER. [...] He looks at Rey. A kindness in his eyes, but there's something tortured, too. He doesn't need to ask her who she is...

Há também o fato de que a novelização de Alan Dean Foster de The Force Awakens contém:

a passage from the Journal of Whills

Parece bastante óbvio, então, que:

Luke went to the island to learn the secret Qui-Gon spoke of, and possibly, even more. Rey's dreams of the island, which we learn of when Kylo interrogates her, were likely planted in her mind by Luke (or Qui-Gon or Yoda or Obi-Wan) through the Force, using a technique similar to the one that Darth Sidious uses to plant dreams of Padme's death in Anakin's mind.

No entanto, acho que Kasdan e Abrams não queriam exagerar no papel dos influenciadores Jedi masculinos de Rey, já que isso tenderia a enfraquecer seu próprio caráter. Ep. 7 é a história da vinda de Rey, muito parecida com Ep. 4 era de Luke. Mas foi melhor para o personagem de Rey que eles escolheram deixar isso para a interpretação do espectador, e deixar os espectadores inferirem e estabelecerem para si mesmos que a própria Força guiou Rey.

Há também a possível explicação de que, se Obi-Wan ou outra pessoa estivesse influenciando Rey através da Força, ela não seria capaz de reconhecê-lo de qualquer maneira, já que ela nunca os conhecera. Afinal, ninguém nunca vê um fantasma da Força de alguém que eles nunca conheceram - o fantasma de Qui-Gon conspicuamente não aparece no final do Ep. 6 ( exceto neste vídeo do YouTube feito por fãs - mas mesmo lá ele espera para aparecer até que o Luke volta está virada, heh). Essa incapacidade de distinguir a voz da Força / fantasma de alguém que você nunca conheceu é consistente com o fato de que a Força e a pessoa são uma , e também é consistente com: l o fato de que:

Yoda's and Obi-Wan's voices in Rey's vision are so quiet and subtley planted that most viewers did not consciously recognize them the first time around.

Faz muito mais sentido quando você percebe que ela (e também Fin, eu diria) tinha uma equipe inteira de Jedi mortos (e um vivo) guiando-os da melhor maneira possível através da Força, como anjos da guarda .