User blog comment:Miskos3/Fairy Tail 541: The Magic of Hope, Review/@comment-4933386-20170626143821/@comment-7418318-20170626172134

Remember that for those within the sphere no time at all passed during the seven years.

I think Lucy is counting on it that Acnologia can only eat magic while he is conscious or rather not frozen in time.

But even if Acnologia's body is only stopped by Fairy Sphere for a few minutes, that might already be enough. After all if the DS manage to defeat Acnologia's spirit in the Ravines, his body will most likely die along with it.