User blog comment:LegerPrime/Natsu vs. Acnologia/@comment-27010145-20151012164019/@comment-24734624-20151012193337

Training to kill Acnologia is the reason why Natsu left in the first place. He said he'd be back when he's stronger. In the last arc, he killed one of the Yakuma Eighteen War Gods, Ikusa-Tsunagi with Fire Dragon King's Demolition Fist. And if Natsu has that, other Fire Dragon King spells, and Dragon Force, then that should be enough to beat Acnologia within an inch of his life. Also, Gildarts wasn't a Dragon Slayer.