I never liked the idea of Wendy should be the one that kills Acnologia. I mean Why? did she has some hatred towards Acnologia?
It was awesome that she managed to win against one of the Tartarus, but if she keeps doing that, her support role will be lost.
As for Erza knocking it out of the sky... I still don't think that's too bad, I mean, magic or physical moves won't penetrate him, but it should still be enough to MOVE an object that's airborne. It's still an object it's not like he has anti gravity that stops him from falling down.