Kyurem-Black Adamant
I feel like Dragon Hammer coverage is redundant with Extreme Speed and would suggest Bolt Strike. Ironically, it has 130 Base Power without STAB, so it is only slightly weaker than Dragon Hammer at 135 Base Power with STAB.
Bolt Strike handles the following- Kyogre, Gyarados, Ho-Oh, and has good neutral coverage against things resistant to your other attacks.
Replace Earthquake with Precipice Blades for more damage, and I also think you could put Sunsteel Strike over Fake-Out, as it is the slowest Fake-Out user and will be outspeed by Triage Rayquaza-Mega, and Fake-Out Diancie-Mega anyways.
Sunsteel let’s it hit Audino-Mega, Xerneas (and Diancie-Mega on a switch in), as well as Shedinja, Fur Coat users like Chansey, and others.
Using Expert Belt would be useful so you can bluff Icicle Plate, and allows you to boost all of your attacks without taking recoil damage. Since Sleep Clause is in effect, Safety Goggles is less necessary.
Groudon-P Adamant
Not a suggestion, just a consideration:
Maybe Electric Surge on Groudon-P to justify the changes on Kyurem-B. It boots Bolt Strike, blocks Sleep, and also gives Groudon-P a stronger move if it wants to use Bolt Strike over Sunsteel. Zygarde-Complete Improofs Groudon.
————
I would give Unaware Zygarde- Complete a Sassy Nature
Defog over Sunsteel Strike, and Core Enforcer over Crab Hammer. Nuzzle over Toxic so Imposter cannot paralyze it.
Please lower all Speed EVs/IVs and use a -Spe Nature on Zygarde-C, as well as Dialga, Slowbro, and Shedinja.
Dialga Sassy nature
Replace Nuzzle with Rapid Spin.
Replace Moongeist Beam with U-Turn so you can now slow pivot to a teammate and heal thru RegenVest. This allows you switch last which pays off for sending in something safely or scouting a switch from your opponent who likely will go first.
—————
Replace Mewtwo-Mega-Y with Fur Coat Cresselia or Volt Absorb Slowbro-Mega @ Leftovers:
Scald, Anchor Shot, Roost, Core Enforcer = Slowbro
to Imposterproof Kyurem-Black, Groudon, Check MMX, and tank some Sheer Force MMY. Use min speed like I advised above.
Replace Arceus with Shedinja which can Imposterproof the foe against Groudon, and lose a threat to the foe.
Give it Stealth Rocks, U-Turn, Endeavor, Extreme Speed. +Atk, -Spe Nature