Natsu only managed to vaporize him, because he just stood there and let himself get blown away, when he was actually trying in their final clash, his body wasn't vaporized, it was badly burned, meaning if he was actually using his power to defend, it would scale to High 6-B, also Natsu over powered Zeref's all out attack in Power, he didn't just Destroy Fairy Heart, he managed to injure, defeat, and incapacitate Zeref when powered by the Fairy Heart and the Ravines of Time, which is bad writing, but has been a fundamental part of the verse throughout the entire series, which is that Power of Feelings makes you OP, that doesn't discredit that he is stronger, also against Acnologia, he was HEAVILY fatigued, after fighting Zeref, while Acnologia was at full power and got another boost in power, also in the official Manga translations of the final battle, Natsu on his own took no damage, from a giant blast of power, and this surprised Acnologia, he then proceeded to get powered by the other Dragon slayers, and he began sweating, then Acnologia starts going insane and wants Destruction and all that madness, and in the original translations Natsu says he can't win on his own, but that was a mistranslation because in the OFFICAL Translations, Natsu says ACNOLOGIA can't win on his own, meaning that Seven Flames Natsu is infact comparable and scales to the ROT Human Acnologia, plus he beat him in one punch, even if, he was frozen still