I knew immediately I wanted to use Mega Gengar + Terrakion because they are a fearsome core together, trapping and KOing loads of pokemon due to their speed and power.
I knew I needed a Landorus-T check if the core was to function properly, as it can outspeed and KO both Mega Gengar and Terrakion in one shot. Zapdos fit the bill well because it also helped with a slight Talonflame weakness, provided a good check to Shaymin-Sky, and learnt Tailwind, to aid vs Rain and scarfed pokemon.
It was at this point I decided to go full Tailwind, and Suicune was my choice of second setter. It provided a Water resist to help vs Rain, Snarl to make special attackers weaker and another solid Landorus-T check as well as a Dragon check with Ice Beam.
Now that I had a good trapping core and pokemon to patch up any weaknesses I had and provide speed control, I decided to dedicate the last two slots towards dealing major damage by abusing the fact that Gengar traps and beats a wide variety of pokemon. Hydreigon was the obvious choice here, as I got an Aegislash check and a way to abuse Fairy types being beaten by spamming its STABs. It also loves Tailwind support to remove the speed issue.
My last addition was Heatran, as it provided me with a good counter to Sun leads (Zard Y and Veunsaur), while beating Amoonguss, which I had an issue with. It also likes the fact that Rock types and Heatran are beaten by Terrakion + Mega Gengar, as it can spam its strong Fire type STAB with more ease.