I would say probably strong enough to flatten a city in a matter of seconds if she really wanted to (then again, most of the dragon slayers could probably pull that off, anyone remember a little port called Harujion/Hargeon...Natsu destroyed half of it in the first episode, by accident). On the other hand, she would probably figure out a better way than that to achieve whatever she wanted. And considering the reactions and respect she gets from Makarov, she must be insanely powerful, but we just haven't seen it yet. And from what I could tell in the manga, Mavis is the one who cast Fairy Sphere in the first place, which was enough to keep out Acnologia. Then if you include her ability to run millions of battle simulations in her head and find the probabilities, and counter strategies relevant to those probabilities, she is essentially impossible to beat. Its like trying to beat someone with precognition, plus an arsenal of very old and powerful magic (she has to have been born a minimum of about a hundred and thirty years before present day if you take Makarov and Purehito's ages into account, Makarov was 40 when he inherited Fairy Tail, now he would be almost if not over 90, that includes timeskip). And we haven't even touched on what kind of magic she specialized in.
