In my reference to Gildarts I didn't mean someone of Gildarts level could kill a Dragon I just said Acnologia was probably already at that level when he was a human and when he learned dragon slayer magic that gave him the power to actually kill dragons.

Of course Gildarts would have no effect on a Dragon but the interesting thing is he actually saw Acnologia's true power, meaning he either pissed Acnologia off or forced him to go serious, but the way Gildarts tells that story it sounds like Acnologia rapped him before he even had a chance.