I've only seen part of the first season of Fairy Tail (up to the end of the lullaby flute battle with the Erigor and that Dark Guild). More specifically, I'm on the episode right after when Natsu and Happy steal the job for the cursed island off the upstairs board at the guild hall.
In the past battle over the Daybreak book, Natsu fought a pair of mercenaries who said that wizards spend so much time harnessing their minds that their bodies are weak, which is their weakness, but after fighting Natsu and his dragon-slayer magic they saw how powerful he was physically.
Exactly how durable is Natsu? Does it scale specifically off of how much fire he has absorbed? Or is his body just naturally tough from training directly under a dragon?