Despite having a very short time to train while actively fighting in a war, Aang still attained high level bending prowess and fought with the best the world had to offer.
And there are different levels to the Avatar State mastery. Korra is nowhere near Aang.
Proof? The only level of mastery I've seen is whether you are able to control it and can go into AS state at will. Nothing I've seen (or at least can remember) suggests mastery makes one more powerful.
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the more spiritual you are, the better you're able to connect to past Avatars and use their bending knowledge. Aang displayed far more impressive techniques in his fight with Ozai than Korra ever did at any time while in AS.
I like this. IMO, earth bending is way more scary and powerful than fire bending is in combat. if it weren't for that episode during Day of Black Sun, I'd say Toph but since the combined efforts of her, Aang and Sokka using both air and earthbending couldn't catch my precious Princess, I'm giving it to Azula. She would figure something out. She is resourceful enough to take advantage of the fact that Toph is blind and unlike Zuko, who Toph would stomp, Azula can shoot jets out to fly