Kartana:
I was told to remove this from my [Gen 8] AG Team because Calyrex-Shadow outclassed it, so I did. But Calyrex-Shadow doesn’t exist in [Gen 7] AG, so Kartana was put back on the exact same.
That's not understandable. How in the world does Calyrex-Shadow outclass Kartana when both do different things? Indeed, you didn't told that but I question SSuperiority for that. Actually, Kartana's much better in Gen 8 AG where it could use Aerial Ace Dynamax as a alternative of Choice Scarf and be able to sweep things along with boosting its Spe. Dynamax doesn't exist in Generation 7 so it arises no use.
Primal Kyogre:
Those EVs are weird on it. You should change it to something like Max Spa Max Spe if you're running a fast attacker, but this case you should replace it with DarkCeus to get a better match-up against UNecro.
Necrozma-Dusk-Mane:
Don't use Knock Off, Photon Geyser or Dragon Pulse. You need a sweeper one that has either a Trick Room Swords Dance or Double Dance Dusk Mane. But, I don't suggest using offensive one right now since you're team has a really, really bad match-up against Xerneas. Change it to SpD set, and make it your Stealth Rocks setter.
Mega-Mewtwo-Y
This? Using this over the supreme AG dominating Mega-Rayquaza is apparently ridiculous. If you had wanted to use it, it must be in a Psychic spam team consisting of Pokemon like Tapu Lele, Deoxys-A which this team doesn't have. Mega-Mewtwo-Y never requires 4 coverage moves; Nasty Plot / Psystrike / Fire Blast / Ice Beam or Recover is the standard set, and you're not utilizing that.
Kyurem-White:
Don't use it in Gen 7 AG. The whole purpose of using it is now in Gen 8 that Primal Reversion has been removed and Kyogre is dominating the meta. Hence, you shouldn't use it. There are a lot of bulky Fairies right now, notably Arceus-Fairy and SpD Dusk mane which give you a hard time. So despite being a Kyurem-White fan and telling to replace it breaks my heart, you still need to get rid of it. It's just C rank back in USUM, it's just not that viable.
Lunala:
It's not a good Z-Move user without Calm Mind. Choice Specs set are considered a lot better for wallbreaking if you don't have the Z-Move slot. So, use Specs Lunala as a breaker instead as we're giving NDM Solgalium crystal.
Tl;dr, use Choice Specs Lunala over Lunalium Z, since you're gonna make Dusk Mane a user of it.
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Replace Kyurem-White with Primal Groudon as your Kyogre counter. It eases up pressure from Primal Kyogre while walling Xerneas, Dusk Mane, Arceus-Fairy while acting as a Stealth Rock setter, meaning you can run offensive Dusk Mane. Here's a set I like using the most:
Groudon-Primal @ Red Orb
Ability: Drought
EVs: 252 HP / 56 Def / 200 SpD
Sassy Nature
- Stealth Rock
- Precipice Blades
- Overheat
- Toxic
Stealth Rocks setter, as well as fading the Magical Deer Xerneas. Overheat hits Physically Defensive Pokemon hard, Rest waves of Status as well your only recovery option. Precipice Blades is 120 BP boosted STAB from 180 Atk hits hard even without any investment. This EV spread almost maximizes SpD while putting some into Def, granting chances to survive some physical hits.
Run Choice Specs set. It's a really good stall and wallbreaker, hard damaging any teams lacking Yveltal or Arceus-Dark.
Lunala @ Choice Specs
Ability: Shadow Shield
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Moongeist Beam
- Moonblast / Ice Beam
- Psyshock
- Trick
This set punishes stall hard. Ice Beam handles Zygarde, Yveltal, Rayquaza. Both Psyshock cripple Chansey, along with Trick. It's really annoying for opponent if they run stall. Note that this has a weakness to Arceus-Dark, so I suggest adding an Arceus-Fairy that can take it as well as Mega-Rayquaza.
We gonna have good physical attackers like M-Ray, Primal Groudon and Dusk Mane so Kartana is uneeded. Get rid of it for Arceus-Fairy.
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It's your main physical wall, a soft check to Arceus-Dark, Yveltal, Mega-Rayquaza, Zygarde, etc. from which the later two are very much threatening to any team. It can also be your Defogger, and is indeed one of best to do so. The set that's used often for that:
Arceus-Fairy @ Pixie Plate
Ability: Multitype
EVs: 248 HP / 244 Def / 16 Spe
Bold Nature
- Judgment
- Recover
- Defog
- Toxic
16 Spe reaches and outspeeds Base 90 Neutral Spe Pokemon. An alternative would be 96 Spe EVs along with Timid Nature for outspeeding Adamant Mega-Ray, but then you stand high chance of getting KOed by a +1 Life Orb boosted Dragon Ascent after Stealth Rocks. Judgment 2HKOes M-Ray or OHKO at -1 SpD or after Stealth Rocks. Toxic cripples other non-steel types, Defog removes hazards from your side.
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As already explained above, get either TR SD or RP DD. Since this team isn't a Trick Room team, it will hinder other Pokemon much. So, I suggest using SD RP. But, in my opinion you should make it a SpD set as you're weak to Xerneas.
Necrozma-Dusk-Mane @ Solgalium Z
Ability: Prism Armor
EVs: 252 HP / 4 Atk / 252 SpD
Careful Nature
- Swords Dance
- Morning Sun
- Sunsteel Strike
- Earthquake
Earthquake hits Primal Groudon as well as Dusk Mane. This set covers a lot of the meta-game—only being threatened by Marshadow for which we have Arceus-Fairy.
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Do I need to explain that when the AG God is under your command, you're gonna use Mewtwo? Plus, what it does is all to type overlap with Necrozma and Lunala, therefore hindering the rest of the team. It literally serves no purpose and reason to use it over Mega-Ray (this doesn't need any explanation!!?)
Rayquaza-Mega @ Life Orb
Ability: Delta Stream
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Dragon Ascent
- Dragon Dance
- Extreme Speed
- Earthquake
This beast. Doesn't need. Any explanation. 120 Dragon Ascent STAB boosted along with Life Orb coming from 180 Atk does nuclear damage, further Dragon Dance means almost nothing in this world can take a +1 Dragon Ascent that doesn't resist it, except the bulkiest of the Pokemon which are still gonna take a hell of damage. Earthquake punishes steel-types as well as rock-types hard. Adamant Nature means 504 Atk, which gonna destroy everything and rip apart teams.
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Primal Kyogre honestly isn't easy to fit. We need a UNecro check as well as a NDM check. Arceus-Dark will fulfill these duties, as well as checking other common Stealth Rock setter in form of Dusk Mane.
Arceus-Dark @ Dread Plate
Ability: Multitype
EVs: 252 HP / 160 Def / 96 Spe
Timid Nature
IVs: 0 Atk
- Judgment
- Recover
- Refresh
- Calm Mind
Dusk Mane and Ultra-Necrozma counter as well as a Bulky Calm Mind Arceus-support forme. Judgment STAB, Recover to heal HP while Refresh gets rid of dem wicked status and Calm Mind for set-up.
I don't think it's justifiable to use Primal Kyogre as viability as this. It provides defensive utility while not being hindering the rest of the team, which is why I recommend using this over Primal.
Final and Updated team
Sorry for a boring answer. Hope this helps though!