Oh come on, it's still a ridiculous stretch. Regardless of natural aptitude, becoming a weapon master takes many years of experience. Training for a mere year with the Faceless Men would make her still a beginner by the end of it, not a master assassin. It's is all simply convenient plot armour, so lets stop trying to justify it as though it is all perfectly logical and reasonable.
But she didn't, that's the point, she was obviously already training as a kid with warriors and teachers who weren't supposed to be teaching her. When she started training with the swordmaster in Kings Landing she wasn't a beginner she already had some skill. Also people who try to pick up something later in life simply aren't as natural as someone who started learning the same talent as a child. So she likely did train for years as kid, you think she became essentially a master archer by 10 or whatever she was supposed to be, because she picked that up over the last three weeks, not the previous few years? Then as said, further lessons from one of the best swords men in the world, then a year of gruelling training with a mystical tint to it? Then there is the fact that she is also naturally small, fast and agile vs big lumbering oafs by comparison. Brienne may be a match for other men in armour on a battle field, but none of them are quick or agile and they really played that in their sparring session, Brienne was slow and swinging with big moves and Arya was dancing around them.
Remember that though the dude got his head a little exploded against the Mountain, one of the reasons he lasted so long and got on top at various points was agility and speed which are both valuable weapons against people with big slow weapons.
To me she trained for basically 8 years in the show on and off with maybe literally the best swordfighter in Westeros and then the best assassin in the known world, which may or may have not come with some mystical powers, and she was learning to fight since she was probably 6-7 years old... she was never a lady, always wanted to be a warrior. Compared to most people in the show who she faced, what was their training?
Training quite a lot with the army, but zero personal training, it's all several hundred people, all frankly a bit crap, training against each other. The fact is armies win by numbers in general, not amazing training, armies lose most of their men and it's a win by attrition, not really training and skill. Most of the people she faced had little to no training.
