I saw someone post he used it because of Acnologia to try and kill him. Then someone say that this is false, he first tried to use it before Acnologia returned to the battlefield. But if you remember, August said it would wipe out the "Entire country." Which Acnologia never left. He was always in the radius of August's spell, just coz we never knew where he was does not mean the spell would not have affected him. Then again, his power MAY have protected him. I'm never one to say just being strong protects you from everything. But this spell is old, Acnologia SHOULD know about it. The fact of the matter is, if it was effecting Acnologia i think Hiro would have shown it to us. Just like he did when Face affected him. In short, August's main attempt with that spell may have been to kill Acnologia, but in the end, it probably would have failed.