CappuccinoJak
[12] Conqueror
depends on what you plan on doing with gyarados. i don't have too much experience with him since mega gyarados doesn't work with my double/triple battle teams since that mold breaker + earthquake = dead gengar. how ever i think he's a pretty good lead pokemon since you can dragon dance and also change your type. you just gotta be careful when you mega evolve because of the different weaknesses water/dark has compared to water/flying. i even had a friend high-jump kick it with blaziken because he thought i was gonna mega it lol.However, I'm wondering who would be good partners for Gyarados with it's mega stone? Magnezone? Jolteon?
tl;dr: i have no idea how to use mega gyarados.