Zacian
Perfect set, no changes needed.
Dragapult
Why would you use it in AG if there are other better 'mons? There is already C-Shadow for you as a ghost type. You might need a defogger or SR setter, so I will suggest you to replace it with Ho-oh. It is a great support for your team.
Ho-Oh @ Heavy-Duty Boots
Ability: Regenerator
EVs: 244 HP / 8 Atk / 252 Def / 4 SpD
Impish Nature
- Defog
- Sacred Fire
- Whirlwind / Thunder Wave
- Roost
Slurpuff
Why Belly Drum + Focus Sash? Belly Drum works only if the HP is more than half, and it cuts off half of its HP. Focus Sash works only when it has full HP. First of all, it is not viable in AG. Xerneas is much more better as Geomancy setter.
Xerneas @ Power Herb
Ability: Fairy Aura
EVs: 248 HP / 252 SpA / 8 SpD
Modest Nature
IVs: 0 Atk
- Geomancy
- Moonblast
- Thunder
- Focus Blast / Substitute
Necrozma DM
Your set is great, except Psychic Fangs < Photon Geyser. It is physical if Atk stat is more than SpA. If you want it to be a dynamax sweeper, you can run Psychic Fangs also. 120 Spe EVs are enough to outspeed Jolly Zacian and Timid C-Shadow after one Autotomize.
Necrozma-Dusk-Mane @ Weakness Policy
Ability: Prism Armor
EVs: 136 HP / 252 Atk / 120 Spe
Adamant Nature
- Autotomize
- Photon Geyser / Psychic Fangs
- Sunsteel Strike
- Earthquake
Marshadow
Run Life Orb instead of Sash, though it's your wish to run either (Life Orb helps in OHKOing instead of doing a 2HKO). Acrobatics doesn't cover many 'mons except Pheromosa, which I guess isn't common. Replace it with Rock Tomb to hit Ho-Oh or Bulk Up to raise your attack and defense.
Marshadow @ Life Orb
Ability: Technician
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Poltergeist / Spectral Thief
- Close Combat
- Shadow Sneak
- Rock Tomb / Bulk Up
Calyrex-Shadow
Mud Shot, Future Sight and Expanding force are not viable. Mud shot has very low power, and Calyrex already has high speed. Future Sight is useless when a Dark type switches in. Expanding force is outclassed by Psyshock / Psychic. Draining Kiss might be an option to hit Yveltal. Nasty Plot to rise SpA to peak. For that, you can run Sash.
Calyrex-Shadow @ Life Orb
Ability: As One (Spectrier)
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
- Astral Barrage
- Nasty Plot
- Psyshock
- Draining Kiss
Pokepaste link: https://pokepast.es/6f4c11ab3fc4eba9
Hope this helps :)