Hello
I am maybe late to answer this question but better late than never Soo,
Rayquaza : Why is he Hasty not Jolly. I am pretty sure It is gonna mega evolve. If it isn't gonna make him. I know that Iron Head is for Fairies and Ice but it isn't viable.I say swap Iron Head for Dragon Dance,one of the best Stat Boosting Moves out there. Brick Break is good for breaking screens but very few even screen nowadays. And Earthquake kills all the Steel your way so those V-Create I say both are great but I personally prefer Earthquake.
Groudon-P : Fire Puch is great but comparing it with Heat Crash makes it look like a speck because of his gargantuan weight of 2204.0 pounds (999.7 kg). Earthquake is decent but you can also swap it for Percipice Blades because it has more power. Granted it is less accurate but is it worth it? (yes). Ancient Power has a chance of Getting a omniboost which is OP but I personally Think it isn't worth as it is a Bulky Sweeper. But seeing your EVs and Nature I think it is mixed bulky sweeper so it's all right. Same applies to Solarbeam.
Kyogre-P : Signal Beam for real. Swap it for Ice Beam or Blizzard as it anyway kills Grass Types bye to its insane SpA. Maybe swap Earthquake for Thunder/Calm Mind?
Darkrai : Leftovers is not good for a sweeper (because of your EVs). Swap it for Psychium Z and swap Dark Void for Hypnosis (as Dark Void lost its 80% accuracy since gen 7) and maybe swap Nasty Plot for Thunderbolt for coverage it is fine anyways.
Xerneas : Swap Leftovers for Power Herb. This is a must or other Scizor/ Necrozma Dusk Mane is gonna switch in and one shot you and swap Dazzling Gleam for Moonblast.
Rotom Mow : It is terrible. If you need a reliable Grass type then:
Shaymin Sky @Choice Scarf
Ability : Serene Glace
Evs : 252 SpA 252 Spe 6 HP
Nature : Timid
- Seed Flare (Stab)
- Air Slash ( 60% Flinch oh yeah)
- Earth Power (Coverage)
- Psychic (Coverage)
If you are need of a reliable electric type then :
Arkoos @Zap Plate
Ability : Multitype
Evs : 252 SpA 252 Spe 6 HP
Nature : Timid
- Judgement (100% Accurate Thunder)
- Energy Ball (Ground Coverage)
- Calm Mind (Boosting Move)
- Extreme Speed ( A faster Better Quick Attack)
So that's it. If I forgot something or made a mistake please respond in comments