How durable is Natsu Dragneel?
Well throughout the series the mercenaries have been proven wrong in just about every fight, no matter the wizard. I'm not sure what makes Natsu so durable, but it seems like every wizard can take a beating magically and physically, most likely based on how much magic power they have (my best guess at least). When you see more fights you will notice that those mercenaries appear to be very misinformed about how most wizards fight.