Universe One happened after August's nuke. With or without Universe One, the fact that we actually saw the attack engulf them meant that they were hit by the full attack. They survived because it was a heat spell and Natsu can negate (to a degree) heat magic. Either way, melt was stronger than any of the nukes August used against other people. None of those other people have died yet. I'm not saying Natsu would beat August. But this idea that a random nuke from August would outright kill Natsu is just silly.It didn't kill natsu for two reason
Universe one
Natsu being able to negate this attack(not all but a little at least)
Without universe one natsu would've been almost unconsious..i can assure you that.
August was not even serious..
And natsu is not stronger than gildarts he can't go df like that..so without df he loses this