I would reccomend using Yveltal or Xerneas. If you don't use legends, then use something like a haunter. If you get it through a trade, that's better because it evolves into gengar, and gengars mega stone is found in laverre city, given by a hex maniac close to a team flare guy. If you can't get a Gengar, Haunter are found in Victory Road, Route 14, Route 19, and though rare, Frost Cavern. Make sure it knows a good ghost type move, like shadow claw or shadow punch.