I always though Katara just really really tried hard in training. And is just a born fighter. I think she's a bit like Harry Potter, where she's a tough opponent as a bender just cos she's a scrapper and would be a tough opponent in any kind of fight.

Like I always love how in her duel with Pakku, where she's clearly massively overpowered, she gets close and tries to actually punch him instead.

And she out thinks Azula more than anything.