User blog comment:IamJakuhoRaikoben/Chapter 466: Assassination, Review./@comment-1359322-20151216073212/@comment-1776824-20151216193233

I think it's more like Acnologia cheap-shotted Igneel and the others with his magic, which allowed him to kill all the dragons. In terms of overall power, as a dragon-human hybrid, clearly Acnologia is on a whole other level; a level that far surpasses probably Zeref himself. However, we have no indication that Acnologia fought and overpowered these dragons, only that he slayed them, and knowing that his magic reaps souls and weakens dragons to the point of death, I'm sure that really makes his job easier. Obviously, as was said, Dragon Slayers gain power by bathing in blood, but Igneel has had time to recover over the years because of the Ethernano in the atmosphere, and his soul was already damaged, so Acnologia could not reap what was already stolen. It ultimately came down to brute strength, and Acnologia lost in that regard, presumably because he let his guard down, however, Igneel also took quite a beating. What we saw was most probably Igneel blindsiding Acnologia, and then Acnologia waiting to hear what Igneel had to the Slayers before resuming the fight. However, as we have also seen, his arm is regenerating, so there's that, too.

This is just my take on things, so no argument intended to commence.