Jeez, give me a break with the attempts at justifying Arya's sword mastery.
Brienne has been training her entire life, has proven herself one of the absolute best fighters in Westeros, and has countless battles under her belt.
Arya had a couple sessions with Syrio and a few months, maybe a year? with the Faceless Men where she spent much more time with stealth and magic and poison etc than with swordfighting -- in fact was she even allowed to touch a sword the whole time she was there? She had one bar brawl with the Hound vs some guys, and one stick fight with the Waif where she got her ass kicked.
Brienne would obliterate Arya under any circumstances whatsoever. The scene was just silly. Arya would not be able to parry a damn thing, she'd get her arm broken if she tried if Brienne was going half-force.
What is the point of training swordsmanship your whole life if you'll still be worse than a 14 year old who maybe picked up a trick or two while training with some face-changing assassins -- assassins, not swordsmen?