For what it's worth, I believe she has a good matchup vs Natsu, you just have to use all her moves and stop trying to play every character the same. The only bad match-ups Leixia has is ones where she is getting severely out-damaged. Which happens a lot, but at times like facing Natsu, it's avoidable. Versus Natsu, the biggest things you have to fear are 8K, and CH WR K/CH 4B. Other than that, keep solid defense, and remember if you're on the ground and see 4A+B or Unblockable, don't get up, but tech roll afterward.
And what exactly can Leixia use against Natsu? Leixia's combo starters and damagers are
all Mid (except Quakestun, but I poo poo that move as a legit combo starter).
Every single one of Leixia's lows is unsafe as heck on block, give no advantage on hit and/or are really slow (i.e. telegraphic).
If you turn it into a slug match of attrition, Natsu wins, hands-down. If you turn it into a reckless game of guessing, Natsu still wins hands-down because if Leixia guesses wrong thrice, she's dead. Natsu can still stand after guessing wrong 7 times as long as none of those wrong guesses was a mid (i.e. "Who the frack cares? Just keep blocking mid").
There is absolutely no way I can see Leixia having the advantage in that match-up. She can't even properly punish the bombs. If she blocks the blue one, she'll lag too much to retaliate with anything more damaging than 6KK. If she jumps it, that's even
less damage (with any given aerial). If Leixia empty hops either bomb, she'll lag too much to punish Natsu for very much damage.
In other words, Natsu can throw out reckless bombs. It's not like she's got a lot to fear in retaliation unless Leixia
predicts them and 3Bs a bomb on
prediction.
How in the world would one of the best punishers in the game not be a bad match-up against a character who
relies on being safe (but whose safeness goes out the window against Natsu)?