Landorus-T, Scizor, Talonflame and Staraptor are all really good partners for Mega Manectric, as they each form a 'VoltTurn' core. Landorus-T is especially good amongst these four as it not only forms a 'VoltTurn' core with Mega Manectric, but also a dual Intimidate core, (just be careful around Bisharp and any other Pokémon with Defiant.)
Other good partners for Mega Manectric include Pokémon who can clear away entry hazards using Defog or Rapid Spin, as Mega Manectric often get's worn down quite easily switching in and out all the time using Volt Switch. Latias and Mandibuzz are good examples of Pokémon that can fulfil this role, thus working well with Mega Manectric, (again just watch out for Bisharp and any other Pokémon with Defiant.)
