This team needs some improvements. Some of the sets are just too sub-optimal, and it lacks any hard Zacian-C counters. I will try to suggest changes for those.
Marshadow
Marshadow @ Choice Band / Life Orb
Ability: Technician
EVs: 252 Atk / 4 Def / 4 Spe
Adamant Nature
- Poltergeist
- Close Combat
- Bulk Up (With Life Orb) / Ice Punch (with Choice Band)
- Shadow Sneak
The moves you're running are fine, but I don't know why there are secondary choices, and why Expert Belt is being used. Expert Belt isn't a great item to have, usually, Choice Band and Life Orb are superior choices as they boost your whole move power, not only for super effective ones. Spectral Thief < Poltergeist because power is generally better, and Marshadow is too frail to steal boosts. Drain Punch and Face Palm are much weaker, so CC is the better option. Bulk Up makes you harder hitting that's nearly impossible to wall. Ho-Oh is rare nowadays, so you can not use Rock Tomb. However, Zygarde is common so Ice Punch is an option over Bulk Up in Choice Band sets.
Groudon
Groudon @ Life Orb
Ability: Drought
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Precipice Blades
- Swords Dance
- Aerial Ace
- Stone Edge
128 Atk / 128 SpA is a bad choice for a sweeper. Regardless, mixed Groudon shouldn't be used at all, as its SpA is only 100 compared to its much higher 150 base Atk. Fire Blast and Solar Beam are bar niche options too, and Serious Nature should rarely be used. Accuracy isn't a problem at all as Dynamax exists, so opt out for Life Orb to boost your power instead of accuracy. Replace Fire Blast with Swords Dance as you really don't need a Fire-type move, and replace Solar beam with Aerial Ace as it becomes Max Airstream under Dynamax that allows it to sweep through teams and become Double Dance sweeper.
Kyurem-White
Kyurem-White @ Utility Umbrella
Ability: Turboblaze
EVs: 40 SpA / 216 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Sheer Cold
- Freeze-Dry
- Fusion Flare
- Roost
Offensive Kyurem-W is a bad choice right now, as offensive Eternatus finds more space. Weakness Policy is also niche choice, and without any bulk it rarely survives a super effective hit. If you need to use Kyurem-W effectively, use the SpD set as it can wall Kyogre pretty well for the team, which is a main threat one should check for.
Utility Umbrella allows you to ignore Kyogre's rain, which means its Water-type attacks would do less damage than their supposed to. The EVs allow you to survive 2 Choice Specs Kyogre's Water Spout. Fusion Flare hurts steel-types like Necro Dusk Mane and Sword Dog. Roost is for recovery.
Yveltal
> Yveltal @ Heavy-Duty Boots
Ability: Dark Aura
EVs: 252 HP / 4 Def / 252 SpD
Careful Nature
IVs: 25 Spe
- Knock Off
- U-turn
- Defog
- Roost
A Offensive Yveltal set might be nice, but you don't need that firepower. Your team needs a Defogger, a Calyrex-Shadow counter which is the top priority while building a team. Both the roles can be compressed quite good by SpD Yveltal, so replace the set with this. Max SpD EVs allow it to wall Calyrex-S the best possible, while Heavy Duty Boots ensure you're not affected by Stealth Rocks. Defog removes hazards, while Knock Off's item removal is handy. U-Turn with 25 Spe IVs is for "slow pivoting" while Roost is recovery.
Zerarora --> Necrozma-Dusk Mane
Necrozma-Dusk-Mane @ Rocky Helmet
Ability: Prism Armor
EVs: 252 HP / 252 Def / 4 SpD
Impish Nature
- Stealth Rock
- Sunsteel Strike
- Earthquake
- Morning Sun
Why are you using this in AG? It's straight up bad, very frail and barely can touch Eternatus. Your team lacks a hard Sword Doggo counter, so we're using the most versatile counter for it, that will set-up Stealth Rocks, punish Fairy-types and a great physical wall thanks to its bulk and Prism Armor. Earthquake prevents you from becoming set-up for things like Zekrom, that resist Sunsteel Strike. Sunsteel Strike is your main hitting move and STAB, that is great against Fairy-types like Xerneas. Stealth Rocks aka hazards are very much required for any good team. Morning Sun is recovery, while Rocky Helmet punishes contact making moves.
Zacian-Crowned
Zacian-Crowned @ Rusted Sword
Ability: Intrepid Sword
EVs: 4 HP / 252 Atk / 252 Spe
Adamant Nature
- Behemoth Blade
- Wild Charge
- Close Combat
- Assurance
Umm I dunno what's the reason of so many choices. Just use 4 wallbreaking options and you're done. Play Rough is unnecessary as you don't really need Fairy-STAB, and SD overlaps already with BU Marsh and SD Don. Sacred Sword is bad as well as you don't mind the drop in defenses, but the lack of power does. Assurance can now be used as we have a Stealth Rocks setter.
Final team
