Hmm Ill try out the 2A, never dawned on me as an answer for the stuff she dishes out.
Also, will require a little hand holding here, but what exactly does good defense mean?
Natsu can definitely overwhelm with mix-ups, and is very fast, but outside of A:6, she has very little options that cannot be 2A'd. When I refer to good defense, I refer to staying disciplined against her and taking the time to block what she's dishing out. There are very few instances where Natsu is positive for no reason, and excluding the times that you can get to her, you can likely punish her, because she's not exactly the safest girl in the game either. Blocking (or jumping) her Ninja Bombs will be half the battle, seeing as how her other PO transitions aren't exactly impressive, not in the least.
She also has many similar strings, with slight alternatives, and in the case of these strings, take the blocks, and punish if they try to marathon. Keep in mind when she's negative on block, and keep in mind, she has lots of things to confuse her opponent, but these tricks of hers are unsafe. Keep your discipline, remember that A:6 (and A6) are -14 on block. Against Natsu, being a defensive player works quite well, and if your opponent has bad defense, get in a good hit then mix them up as hard as you can. Do NOT forget, if a Natsu tries to marathon you, 2A, Grab. If they try to duck it the next time, launcher.
Leixia has the advantage in this fight with good defense. Stay disciplined, and stick to your guns, and you'll be fine.