KyogreF4N HO

The team is straight up Hyper Offense. You will lead with Deoxys-Speed in almost all cases, except against other Deoxys-Speed (because you're slower), as well as Magic Bounce Pokemon such as Mega Diancie and Mega Sableye. The objective of Deo-S is to lay down hazards, at least Stealth Rocks, and 1 layer of Spikes if possible. It can also pick up some unexpected KOs on Marshadow thanks to Psycho Boost, and the EV spread is made to live a Shadow Sneak from Life Orb Marshadow, which could otherwise stop it from doing it's job. Rocky Helmet is useful against Smeargle, because you can Taunt it to prevent Sticky Webs (Sticky Web is a big hindrance if it ever goes down), and Psycho Boost will break Smeargle's Focus Sash, making it very difficult for it to set up Webs or Rapid Spin your own hazards away.

Then, you have several setup sweepers that can severely break the opponent's team to the point where another member can clean up. Double Dance Dusk Mane is an amazing late game sweeper, especially with hazards up. It can indeed set up on a big portion of the tier, examples being most Fairy and Steel-types. Steel/Ground coverage is very good, and the Z-move ensures a KO on most Pokemon at +2. It's also an extremely solid stallbreaker thanks to Swords Dance and the Z move, being able to OHKO Mega Sableye, Zygarde or even Ho-oh at +2.
Arceus-Ground is so vital because it can set up on Primal Groudon, which is an immense threat to any team in Ubers. Ground/Rock coverage is one of the best in the game and Recover gives you that useful longevity, allowing you to boost on support Arceus variants lacking Will-o-Wisp or super effective Judgement. Maximum speed ensures you outspeed support Arceus, and at least tie with Mega Salamence and offensive Arceus forms. Watch out for Marshadow though, because it can steal your boosts and greatly damage your team with them. Scarf Yveltal is also able to pick up a KO on your Arceus with Foul Play after a Swords Dance.

Mega Salamence provides additional support, being another check to Primal Groudon, thanks to intimidate and amazing natural bulk. With Dragon Dance and 3 attacks, it can dish out enormous amounts of damage immediately, and pressure the opponent so he can't Defog away the hazards. Facade makes it an amazing status absorber (especially burn), because it gives you an extremely powerful STAB option without recoil. Double Edge is your most spammable STAB move, knocking out threats such as Xerneas, Yveltal and offensive Arceus after a single boost. Earthquake is used for coverage against Steel/Rock/Electric types that resist Flying STAB. Moving along, Mixed Rock Polish Primal Groudon is a crucial addition to the squad. After a Rock Polish, you outspeed every threat in the tier, being able to hit them with unbelievable power and coverage. Hidden Power Ice catches some threats such as Mega Salamence, Zygarde and Rayquaza, which could otherwise check it and prove to be very annoying against the rest of the team. On top of that, you have your dual STAB options in Fire Blast and Precipice Blades, being able to dish out strong hits both on the physical and special sides. Finally, Geomancy Xerneas is a staple on most offensive teams, due to it's ludicrous efficiency offensively and defensively. It's the best way the team has to pressure Yveltal and Marshadow, so it is often used to obtain momentum with double switches, letting it's checks take hazards damage, often opening up the door for a late game sweep. Hidden Power Fire allows you to hit Steel-types more effectively, especially threats such as Ferrothorn and Scizor. Thunder allows you to hit other checks, such as Toxapex, Celesteela and most notably Ho-Oh.

Despite Xerneas' presence, the team struggles against Marshadow, it is faster than every pokemon on the team (except Deoxys-Speed of course) and can deal damage or steal boosts. If you find some room to set up in the speed department, then it becomes a much smaller threat, being outsped and only being able to throw Shadow Sneaks off at you. Scarf and defensive variants of Yveltal are also a clear issue. Make use of Xerneas doubles, predicting it's switches, and manage to keep rocks up no matter what, so you can keep pressuring it. Opposing Xerneas are also very threatening, but Dusk Mane will survive any attack if kept healthy. Keep it healthy to avoid getting swept! Your Primal Groudon can't check it as well due to the offensive nature of the spread. Arceus Water can also be very annoying, but Dusk Mane can set up on it if it lacks Will o Wisp, which mitigates the problem. Mega Salamence can also set up on it if it lacks Ice Beam. It's tough to prevent it from removing the hazards with Defog, so you might often have to pull some risky doubles to Xerneas in order to pressure it. Keeping hazards up the whole game, or at least for the most part of it is really the key to success with this team. If your opponent gets a Defog off against you early on, your double switches are less rewarded, and you miss some crucial chip damage (on support Arceus for example) that can prevent a sweep.

Enjoy! :)

KyogreF4N