Finally my man Pyrrhon with the RMT. Congratulations, Kamina. Another lord squad for you to steal!
Anyway, I don't have much to say about this team. It does look really cool to use, though. I'll tweak the EVs a bit for better use. (Optional EVs, really.) For Heatran, I suggest going to a Magma Storm+Taunt, so you can help the Stall weakness. Although by no means is it an autowin for you, it should aid you better than your previous set. If Flash Cannon is only for Clefable, I'd drop it for Taunt, since fairies can't do anything to you anyway. The offensive fairies kill you with Focus Blast(Gardevoir)/Earth Power(Diancie)/Earthquake(Altaria), and they outspeed you anyway, so hitting those won't matter in the long run. Defensive fairies lose. Gonna EV Heatran so Chansey still only 5HKO you with Seismic Toss, while investing in offense so Magma Storm doesn't do jack shit to Mega Sableye. Gastrodon, I just invest in all defense so you can avoid the 3HKO from Choice Specs Keldeo, assuming no Stealth Rock and you are at full HP. I'm not sure what the sp def EVs do anyway, so can you please explain what it does? Relaxed Nature instead of Impish, just so scald doesn't get weakened, even though it probably still won't do anything anyway. It's not like the speed drop really hurts you, since you're gonna set up with Curse regardless.
To help solve the somewhat problematic Mega Metagross and Bisharp problem, I go for Counter>Iron Head, on Skarmory. Also let's face it. Diancie-Mega isn't staying in on Skarmory anyway and Thunder Wave CM Clefable beats Skarm, with Iron Head or not. Also, I'm giving Skarmory a spread of EVs: 248 HP / 176 Def / 60 SpD / 24 Spe, so you can take a Draco from Latios, and then live the next Draco or the Hidden Power Fire, right after. Other changes are extremely small, but I hope they work out for you, even if this team isn't that serious.