I'd say it would be close, but Mei would. Her lava would stop A in his tracks, negating his speed, and even if that were to miss, her boil style jutsu would get him! This would be close. Plus, we haven't seen her other jutsu!
If her Lava release is able to pin A down then she would win, otherwise no, The Raikage is just too fast, he wouldn't have time to be affected by the Boil release.
Of course this is just based on what we have seen so far.
I agree with all the posts above A is too fast and strong for Mei.. unless she had prep time to use her Boil Release in the area and make it very acidic before he got there but if not A still wins