None of them are great warriors. This is why Jamie clowned the hell out of him. They got to where they are with trickery and dirty handed bullshit. Clearly Arya picked up more than a little bit of skills from her time in Bravos. She wont be cutting down 20 men by herself. She wasnt learning how to be that, she was learning how to you at your most vulnerable. Walder Frey wasnt expecting a damn thing. He was smug as hell.