What I don't understand is why he is suddenly so much more powerful. Wasn't the entire origin of this manga that he (and practically anyone else) wasn't powerful enough to defeat an "S" Rank disaster of a monster? Or was that first dragon something above "S" Rank? "SS"? "SSS"?
So far as I recall, and I don't claim to have a good memory), he has maintained his skill level (or it has diminished if his former partner is correct), so if he can kill an "S" Rank now, he should have been able to "do something" back in chapter 1...
/Sigh
I will actually go and double check this when I'm not on my phone.