The same way that Luke Skywalker went toe to toe with the strongest Force user ever based on 5 minutes with a droid and lifting some rocks.
The idea you need some long term training in the Force is prequel bullshit and is now, thankfully, being roundly ignored.
Not sure where you get Luke only training for 5 mins. Unless there's some specific timeline elsewhere i'm unaware of, most people got the feeling he spent more than 5 mins with Yoda etc. Also in the early films there was a perception given that 'the force' was firstly an inate ability passed on through a bloodline (otherwise what are the odds all the Skywalker clan have the ability), and secondly they became more powerful by training, guidance, mentoring and a honing of the skills over time (hence the Obi-wan/Anakin & Yoda/Luke padawan stuff etc).
Now with Rey's character arc it has shown all of this to be bullshit. Basically now anybody can be born with it, there's no discernment of how powerfully they'll be able to wield it plus the abilities it conveys literally appear at the flick of a switch. Things like Rey being able to fight like a master with a light sabre after hardly handling one previously is partly the reason the 'Mary Sue' accusations get thrown around.
You obviously seem to have some sort of downer with regard to the earlier films/episodes, so have no problem with seeing their legacy trashed to enable the franchise/storylines to move to new ground, but i find it understandable why some people are uncomfortable/unhappy with all the disconnects & inconsistancies.
I thought the film was okay/good and a steady 7/10. What i find more interesting though at the minute is the social discourse/debate surrounding the film. You've got the fanboys/EU's etc who aren't too happy, through to those who seem to think it's the second coming or the new marvel of modern cinema and won't accept any criticism about it.
Add in to the mix a Rotten Tomatoes critic rating of over 90% compared to an audience rating of around 50% (most other rating sites fall around the 3-3.5 out of 5), along with allegations of rating fixing from both sides, then numerous articles defending/trashing it for various reasons and it all becomes more entertaining than the film.