So I started with Gastrodon and picked Gardevoir as my mega since it struggles against Steels, which Gastrodon checks, and Gardevoir can provide Trick Room support, which Gastrodon appreciates. Next I added Heatran to check Grass and Steel types, specifically Amoonguss and Aegislash. Then I added Amoonguss mainly for Trick Room Spore, but also since it pairs nicely with Heatran and it completes a F/W/G core. I added Landorus because I was pretty weak to Steel-types, and Intimidate is nice. It also gives me an offensive check to Kangaskhan. Finally I added Gengar because I wanted a mon immune to Ground for Landorus, and WoW + Taunt is great support for Gardevoir, who doesn't appreciate Spore or strong physical attackers.
The Gastrodon EVs are from n10sit's analysis in progress. The Gardevoir EVs are from Kyle's post in the EV spread compendium. The rest of the spreads are standard.
You could run a min Speed Eruption Heatran if you wanted, but I didn't want to be so reliant on TR. Icy Wind is a cool option on Gengar, but then you have to get ride of Protect or another supportive move which I wouldn't particularly want to do.
tl;dr GLOP