Hi! That's a nice BO style team, but I'll like to perk it a bit so that it becomes better.
I'm not a big fan of BO builds that lean a little towards HO, but over here, I'll try to keep the main Rayquaza-Mega + Kyogre-Primal offensive core intact and then suggest some changes.
Rayquaza-Mega
Make it non-Shiny. Shiny is just a giveaway that you're not a V-Create variant, which eases mind games for the opponent. Seriously, do looks matter that much? I guess not. Otherwise, the Rayquaza-Mega is all good.
Rayquaza-Mega @ Life Orb
Ability: Delta Stream
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Dragon Ascent
- Dragon Dance
- Extreme Speed
- Earthquake
Extremely standard, you know about this.
Darkrai --> Ho-Oh
Darkrai, after Dark Void's nerf, is extremely bad in USUM AG. It's deadweight; one Hypnosis miss and the opponent shuffles you with some phazing move. Your team is getting 6-0'd by Kyogre-Primal + Xerneas, too. Ho-Oh acts as a check to Xerneas, and makes a sweet balanced core with the Groudon-Primal and Arceus-Water (we'll be changing the FairyCeus, read on). If we talk about Ho-Oh cores, though, the Arceus-Ground + Ferrothorn + Ho-Oh defensive core is extremely efficient against most offensive cores in USUM AG, but that'll actually change the whole look of the team to a bulky balance, so I'm not doing that.
Ho-Oh @ Safety Goggles
Ability: Regenerator
EVs: 248 HP / 204 Def / 56 SpD
Relaxed Nature
IVs: 0 Spe
- Sacred Fire
- Toxic
- Defog
- Whirlwind
This Ho-Oh set will do the job of both the Zygarde and a Ho-Oh. How? With Safety Goggles + Whirlwind, Smeargle ain't gonna be no baka doe here. I prefer this over Goggles Haze Zygarde as Ho-Oh can Defog away Smeargle's hazards too, which is another bonus. Rest everything is self-explanatory, 248 HP + 56 SpD so that +2 Thunder from Xerneas doesn't OHKO from full, rest is dumped into Defense. Ho-Oh doesn't mind Toxic much, too, as with Regenerator it can simple heal off the damage, so you can switch it into random Toxic's or stuff.
Groudon-Primal
Sadly, ParaDancer and its forms were better in ORAS AG than USUM AG. USUM is a faster metagame, with threats that can finish off a Groudon-P more quickly, and with more of Rayquaza-Mega and especially Zygarde-C, ParaDancer / any retake just isn't that good. I'd like to replace it with a SpD set which is Groudon-Primal's best USUM set and I'll hear no argument.
Groudon-Primal @ Red Orb
Ability: Desolate Land
EVs: 248 HP / 8 Def / 252 SpD
Sassy Nature
- Stealth Rock
- Precipice Blades
- Dragon Tail
- Toxic
Another standard Groudon-Primal set here. Max HP / SpD is for Kyogre-Primal and SupportCeus formes, and actually this set is a big reason because of which Groudon-Primal is now S- in the Viability Rankings. Stealth Rock + Toxic is extremely irritating for a majority of SupportCeus formes, as they'll be having more pressure while Defogging away the Rocks. Precipice Blades is the chosen STAB to hit the aforementioned foes, while Dragon Tail is picked as the last move to hit Rayquaza-Mega hard on the switchin / whenever. Groudon-P also covers for Ho-Oh's Water-type weakness, and as I've stated before, Groudon-Primal + WaterCeus + Ho-Oh is a heat core.
Arceus-Fairy --> Arceus-Water
Arceus-Fairy is one of the best Arceus formes, but now we needed something for Groudon-Primal's, especially Swords Dance + Rock Tomb versions. Arceus-Water is a cute check to them, and also fills in the role of a Defog user, as Ho-Oh takes 49 from Rocks :[. Arceus-Water does everything that Arceus-Fairy does, bar the factor thatt FairyCeus doesn't instantly lose to Mixed Rayquaza-Mega. It also checks Swords Dance GroundCeus (which FairyCeus can't do efficiently) and Marshadow, two extremely threatening wallbreakers. I'll be doing a PhysDef set so that it checks the above mentioned Pokémon beautifully.
Arceus-Water @ Splash Plate
Ability: Multitype
EVs: 248 HP / 244 Def / 16 Spe
Bold Nature
IVs: 0 Atk
- Recover
- Toxic
- Ice Beam
- Defog
Standard again. Actually if you want to win on ladder, you'll usually be going for standard sets... Anyways, Ice Beam hits Rayquaza-Mega, Arceus-Ground and others for super-effective damage, Toxic is important here as it pressurises Groudon-Primal and all, and 16 Speed EVs let it outspeed neutral max invest base-90's, like the Primals. You can do Water-Z for the giggles but meh.
Kyogre-Primal
All is good here. With doubles to Rayquaza-Mega, you'll be pressurising the blobs, Ferrothorn and Groudon-Primal enough so that the Kyogre-Primal becomes a wincon late game. Rayquaza-Mega + Kyogre-Primal is the main offensive core of the team and I don't wish to alter it in any way.
Zygarde --> Yveltal
As we already have Goggles Ho-Oh for Smeargle and co., and Arceus-Water for Groudon-Primal's, Zygarde, even though doing a lot of things here, might just be replaced. After fiddling a bit with things like Tyranitar and Necrozma-DM, which form balanced cores with Kyogre-Primal, the team became a little balance-ish, and I don't wish to change the main style of BO. In the end, I came up with Scarf Yveltal, which gives us speed control, checks Necrozma-Ultra, Necrozma-DM and Gengar-Mega, which actually sit on the team. Also, the team's highest speed was 329 (Rayquaza-Mega), which is pretty slow, so Yveltal, which makes a nice balanced core with Ho-Oh, fits nicely here.
Yveltal @ Choice Scarf
Ability: Dark Aura
EVs: 176 SpA / 80 SpD / 252 Spe
Modest Nature
- Dark Pulse
- Oblivion Wing
- U-turn
- Foul Play
A bit un-standard set here. 80 SpD lets it survive the 2HKO from Ice Beam from SupportCeus formes and retaliate with Dark Pulse. Oblivion Wing is a mandatory STAB which hits everything that Dark Pulse can't hit, recovers HP and OHKO's Marshadow. U-Turn is a great move which gains momentum. Foul Play is preferred over Toxic here so that we can OHKO Necrozma-DM and Extreme Killer. An alternate spread of Modest with Max SpA / Spe can also be ran, it's a choice.
Final Team [Importable]
That's all, hope it helped!