Mega Gallade and Bisharp together form a great core, as they can dispatch each other's checks.
Because of this, you would need something to absorb Will-O-Wisp or beat Talonflame or even provide a nice Fairy resistance. Heatran is excellent for this role.
It's important that you have entry-hazard setters to whittle down the opposing team (Mega Gallade forces a lot of switches). For this reason, Landorus-Therian and Garchomp are good options, as they can also deal with Latias and Latios for Mega Gallade. If you decide to use Landorus-Therian, Rotom-Wash is a solid choice to make a Volt-Turn core with it.
Klefki also is a good partner because it provides Thunder Wave support, allowing Mega Gallade to clean up.
Fairy-types, such as Clefable and Sylveon, are also important to deal with Mega-Sableye.
So, here's the list of good teammates:
Hope I helped!