User blog comment:Miskos3/Fairy Tail 536: The Flame of the Dragon's Roar, Review/@comment-7418318-20170520220908

Theroy time!

Maybe Natsu didn't beat Zeref with the power of feelings, but Lucy gave him the necessary ability to neutralize Zeref by rewriting the book of E.N.D.

With the whole "If Zeref dies, so does Natsu" thing it would stand to reason that they are connected. If Lucy could rewrite this connection to spread the power of Fairy Heart between them or maybe even enable Natsu to absorb FH from Zeref it would explain how Natsu could overpower Zeref even though Zeref had an infinite source of magic.

Or maybe the ship and Acnologia crashing into the ravines of time while Zeref was connected to them messed him up and Natsu hitting him at the same time was just coincidence...

Yeah I know, I'm probably overthinking it, but either way would be a lot better than "The power of his emotions burned even time itself."