Yeah, my biggest problem with it is that Brienne was a match for Jamie, and I don't see it making sense that Arya could be better than a two-handed Jamie. But at the same time, as you point out, there are tons of instances in the book and show where unexpected techniques trump power, skill, and strategical advantage. Syrio, Bronn, Strong Belwas, Oberyn, Rob's victories, the battle of the blackwater, the Sons of the Harpy, and lots of times with Stanis and the Mel. Therefore, it is easy for me to see a scene like this happening in the books. I doubt Brienne has much experience fighting against such a foreign style as she was taught to fight honorably, and has shown difficulty matching inferior fighters willing to fight dirty (Bronn mocks her training for this in the show). As presented, Arya seemed a bit too confident, but that is kind of where her character is at right now (hubris comes before the fall). It would have been cooler if they had brought Arya's Warging into the show though and she had used that to pull off something unexplainable in the fight.
