Hydreigon is an extremely powerful wallbreaker. As said above, the way to go is Specs Hydreigon + Screech Dugtrio, mainly to trap Chansey and Clefable.
I decided that Custap Dugtrio is the set I want to use because my team lacks hazard removal. 21 HP IVs ensure Dugtrio is at 1HP after 2 Seismic Tosses so if Screech misses you could still hit hard with Reversal.
Both Dugtrio and Hydreigon have a hard time switching into Fairy types and fast psychics so I thought
Mega Scizor would be a good idea Escavalier could be... nice? It traps Lati@s with ease (unless they have HP Fire but it could still survive one) and it also eats any Fire type attack from fairy types and hits back with an Iron Head. Max HP for bulk, speed EVs allows it to outspeed uninvested base 30s like Amoonguss and Slowbro and rest into attack.
I wanted a physical attackers check and a Stealth Rock user that doesn't lose momentum all the time so I decided Landorus-T is a great idea here. It sets up rocks, checks physical attackers and brings Hydreigon in with a slow U-turn.
Since Chansey is trapped by Dugtrio and I needed a stallbreaker I thought Clefable could be a great idea. I wanted Life Orb instead of Leftovers for stall because I already have Landorus-T for Mega Lopunny and Escavalier for Latios.
Now that I had 5 members and still didn't use my mega slot, I was looking for a mega to complete the team. Mega Manectric looked cool because it completes a VoltTurn core with Landorus-T and Electric is a really good offensive typing in competitive.
http://replay.pokemonshowdown.com/ou-428105245