reeeeeee fine
AP:
So, it's Natsu who solidly overpowered Etherios Mard Geer who is stronger than Base Mard Geer who is superior to 578 megatons vs Charizard who is undamaged by 326 megatons and is superior to 531 megatons. So, they start off pretty comparable. Then, Natsu's FDS means he gets stronger when eating fire and this is especially true when the fire comes from a strong person. So eating Charizard's fire would make him stronger. There's also Dragon Slayer's Magic buff against Dragons. Before anyone says anything, Yes, I know that Charizard is Fire/Flying, not Dragon. However, that doesn't matter. What matters is if the opponent is considered a dragon/bears resemblance to one which is what Charizard would look like to Natsu. The Dragon Slayer Magic buff let's
Low 7-B+ characters harm characters comparable to
7-A+ character.(No, it's not an outlier or PIS, it's directly stated that happened due to the Dragon Slayer magic.) So, Natsu would have a solid AP advantage.
Powers and Abilities:
Since Natsu resists fire, Charizard's fire attacks aren't going to have any effect on him and will just be eaten for amps. Natsu can probably eat the lightning off the thunder punch too and that would let him use lightning attacks as well which is what Charizard's weak to. Charizard's also resistant to fire and would be pretty alright with Natsu's fire too. Charizard's Blaze is also pretty much a non factor, since it raises the power of it's fire attacks so Natsu would just eat those and get even stronger. Because there's a misconception about this I've seen sometimes, Natsu is not rendered powerless without his flames. Not only does his regular fists still hit around the same level of strength, but he's adapted to fighting his opponents that his fire doesn't work against by doing other things
like using his flames behind his fists to use as propulsion for his fist to hit the opponent without the fire connecting. Charizard still has his other moves to fall back on like his Mega kicks/punchs, Wing attacks, Shadow Claw, etc.
Skill/Experience:
Natsu's fought opponents that are
hundreds of years old by this point. He's been trained for most of childhood by
Igneel who's also hundreds of years old. He's also skilled enough to fight two people for a few minutes and then memorize their fighting styles and the patterns that they make when they move. In comparison, Charizard has more of a general species profile than a specific individual with feats. Generally, most pokemon are trained to and fight for most of their lives with Charizards being known for seeking out challenges.
Weaknesses:
Natsu can't eat his own flames. Charizard's are also known to be proud and overconfident. Charizard will instantly die if it's tail flame is put out.
Conclusion:
I'm going to say Natsu takes it for these reasons. The fire attacks will be no selled so it's mainly going to be a CQC battle. You could argue that Charizard could stick to the air and attack him with Dragon Breath which isn't fire, but I disagree that a Charizard would try winning with a strategy like that. It would mean it doesn't think it can take a human in close quarters and I feel like they're too prideful for that. I mean, they even fight legendaries who vastly overpower and outskill them. So, I think Natsu would beat Char via amp from the first few fire attacks and DS magic amp which should still apply with the moves even if the fire directly doesn't connect.