For the most, part, Mega Gyarados is outclassed by its normal form. The only real niche MegaDos has is hitting Rotom-W switchins with EQ. Those 2x weaknesses hurt a lot more than you think, especially that Fighting one. Part of what made Gyarados so great was its defensive typing, which is moot in Mega Form.
Yeah... Sure he had a 4x weakness to Electric, but that and a 2x to rock, he was difficult to deal with, on top of his fairly beefy Sp. Def and AWESOME movepool. The trade into Dark w/ Mold Breaker lets him hit levitate Pokemon w/ Earthquake and some better stats... but a trade off he didn't need. I'd say he's better off with a Life Orb DD or Assault Vest set up on Moxie rather than going Mega. It IS nice that his defences are both above 100, but there are better slots to put that Mega Stone.
[EDIT] However, a strategy I hadn't considered before: Moxie and DD, attack like normal, and when they pull out their Electric user or something w/ Levitate, mega evolve... I always forget you don't have to Mega Evolve right away.
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