Mega Garchomp is great and all, but I don't think it's something you should be too engrossed about. First of all, it's speed stat gets lowered from 102, to an average speed of 92. Many people don't realize it, but speed is one of the most important stats of the game. There are tons of Pokemon that have speeds in between 102 and 92, and that puts Mega Garchomp at a little at a disadvantage. Also, Garchomp has the more useful ability of Rough Skin, which could damage the foe on contact. Mega Garchomp has the ability Sand Force, which only raises Ground, Rock, and Steel type moves in a sandstorm, which means the ability can only be activated by a Pokemon with the ability Sand Stream, like Tyranitar or Gigalith. Mega Garchomp may have a high attack stat, but a Garchomp with a Life Orb would overtake Mega Garchomp on that. If you insist on having a Mega Garchomp, it's not like it's horrible, it just probably shouldn't be your top priority if you want one.