You wanted to know which set pushed Mega Banette to C, here's the original nomination by Ben Gay, and should contain the explanation you were looking for as well as the set:
http://www.smogon.com/forums/threads/oras-ou-viability-ranking-thread-v3-read-post-1054-page-43.3536420/page-34#post-6246951
http://www.smogon.com/forums/threads/oras-ou-viability-ranking-thread-v3-read-post-1054-page-43.3536420/page-34#post-6246951