This was the original core I started with. I really love Kyurem-Black, as it's really hard to switch into, and it does really well against Balance teams and fairly well against Stall teams. Mega Manectric struggles with both of those archetypes, but it's able to do really well against HO, which is something Kyurem-Black doesn't do too great against. Kyurem-Black also gets rid of fat grounds and grass types for Mega Manectric, so that made me really wanna go forth with this one.
Neither member of the original core could defeat Clefable, so I decided to add Talonflame, as it's able to really shut it down (which is really fun to do), and it also gave me a stallbreaker, which was great seeing as how stall was the least prepared playstyle to face for the original core.
Sand offense looked really annoying for my team to face, and I decided to add a revenge killer in Azu, since it's able to defeat some mons via Aqua Jet, and more notably check some mons on rain teams like Kingdra. It also gave me something to send in to handle Zard X, as Aqua Jet + Flare Blitz damage would sometimes be enough to take it down, and if not, then Talonflame could finish the job if necessary.
At this point, I felt like I needed something else to help out with Rain, Sand and Keldeo. Luckily Celebi fit the bill, and it gave me a stealth rock user. It also gave me something for speed control and taking scalds/status conditions, which I felt really threatened my team at this point.
I didn't have anything for Mega Diancie nor anything for switching into Latios barring Azumarill (who wouldn't wanna take a Psyshock/Thunderbolt anyways), but I also needed hazard removal for Talonflame, and that's where Skarmory really fit in.