User blog comment:IamJakuhoRaikoben/Sorcerer Magazine, Issue 45: September 2015/@comment-25111584-20151003185436/@comment-4829046-20151004071549

@anon I think you are also on the assumption that the other S-Class didn't train while Natsu was on his journey.

If you are saying that Natsu has what it takes to defeat any of the S-Class Mages in 'Fairy Tail' by his lonesome, then you are mistaken. They all respect him, but pretty much all instances where they were in a bind were because they were at a disadvantage. With Natsu, there is no real disadvantage to speak of. Laxus for instance, who Natsu may put at a disadvantage due to use of Lightning-Flame attacks, has still demonstrated that he is far above Natsu. Being able to output attacks of much greater magnitude and with lesser effort.

Natsu is more specialized at delivering the final blow. He is able to fight the big baddies due to a variety of factors, but these factors disappear when facing a S-class from FT guild.