User blog comment:Miskos3/Fairy Tail 529: Teacher, Review/@comment-25111584-20170331224055/@comment-30462872-20170402113702

Igneel was described by the other dragons to have 'fought from the very brink of death', whereas they had more life in them which is why they took out FACE. The fact he took off Acnologia's arm is amazing as it is, and with brute strength alone. Even Acnologia himself was shocked when that happened. That shows just how weak he was at that point.

I mean, Igneel could barely summon Roars, Iron Fists and a Talon to combat him, and they pretty much did jack all. Not once did we see him use a Fire Dragon King spell, not even Demolition Fist which Natsu was able to pull off alone. Keep in mind Natsu is restrained by being human, not a pureblood dragon like Igneel, who pretty much lived and breathed magic.

I'd go as far to say Natsu would have fared better against Acnologia than Igneel. Overall, from my perspective Igneel was weaker than the Natsu at that time when he confronted Acnologia. Natsu had fought dragons previously, even managing to take down (and possibly kill) Motherglare.

That my friend, is why I believe Acnologia bested Igneel, the Fire Dragon King, despite losing an arm.