>>20844200mega banette is not particularly powerful, so the question would be "how to beat it without loosing a pokemon to priority destiny bond"
Talonflame and priority users that are faster than banette could take it down without the risk, but except for the smogonbird most of them are risking a burn with will o wisp. It is unlikely that m-banette will be seen in OU and thus talonflame is not a main issue.
I can't right now think of a perfect pokemon in RU to counter it really, anyone has ideas?