This team itself is basically a team that focuses on each Pokemon taking at least one pokemon down - could be counted as a one-for-one team. No defensive switch in is required, as each Pokemon covers their own weaknesses, for the most part. Explanation for individual Pokemon below, because I find it helps explain the individual uses of each Pokemon.
Marshadow is the team's counter to opposing Marshadow and any setup Pokemon, such as Power Herb Geomancy Xerneas or Dusk Mane Necrozma. Close-Bat is for a Fighting STAB, and Spectral Thief is its way to steal stat boosts. Shadow Sneak is a priority move that is also STAB. The last slot can be given anything, but I gave it Protect solely to let setup users set up, so they get stat boosts, and to also scout for opposing threats to Marshadow. Focus Sash is given as so to allow Marshadow to survive a hit before stealing stats - otherwise, Geomancy Xerneas wouldn't be countered, or any other setup Pokemon. Given EVs are to outspeed certain Pokemon (383 base) and to hit as hard as possible with its attacking moves.
A variation of Extreme Killer Arceus is used as the team's late game sweeper and all around priority attacker. It is given Extreme Speed and Swords Dance because that is the base for Extreme Killer, and allows for quick yet powerful sweeping. Shadow Claw is used to counter Ghost types, who are immune to Ex-Speed, but can also be used if it is preferable to Ex-Speed (i.e. Cresselia, Dusk Mane Necro, etc.). Finally, Perish Song is to force switches if an opponent has a harsh sweeper that the team would otherwise lose to. Given EVs are to take multiple hits (Swords Dance) and Ex-Speed as hard as possible. Chople Berry is to allow it to live at least one Fighting-type hit.
Dusk Mane Necrozma is used as the team's 1-time nuke and a second counter to Xerneas. It has Sunsteel Strike and Solganium Z for the said nuking, as well as a decently damaging attack once Searing Sunraze Smash is used up. Swords Dance and Rock Polish are setup, though Rock Polish can be replaced with Autotomize if it is more preferable. Do note that Autotomize lets Heavy Slam do more damage. Morning Sun is a relatively reliable recovery option, as it gains 50% health unless under Primordial Sea or the super-rare Sandstorm or Hail. Again, EVs are to hit hard and fast, and to maximize output from Swords Dance and Rock Polish.
Kyogre is the team's special attacker, with hard special offenses. Blue Orb activates Primal Reversion. Origin Pulse is Kyogre's strongest attack with a fixed damage output - it is more preferable to Water Spout in the long run due to Water Spout's lessening damage at lower health. However, Water Spout is viable. Thunder is for perfect accuracy and an attack for opposing Water-types. Ice Beam is to counter Primal Groudon and Rayquaza, along with further coverage. (By "counter Primal Groudon", I mean to actually hit it.) The last move is open to the user's choice, but Protect is given for scout of hard-hitting opposing moves, and a way to PP stall opponents.
Groudon is the team's physical attacker. Red Orb for Primal Reversion. Precipice Blades is a hard-hitting STAB move, though with reduced accuracy. (Earthquake can be used if one feels it is more reliable.) Protect, again, for scout, and PP stall, but especially to see what move an opposing Primal Kyogre has, when under Primordial Sea. Fire Punch is chosen over Eruption, not only because of the damage decrease, but also because this set is physically biased, and Eruption is special. Fire Punch also has a burn chance, which is low, but exists. Thunder Punch is coverage, primarily against Rayquaza and Primal Kyogre when in Desolate Land. Given EVs are damage, survival, and a tiny bit more survival on special hits against Groudon.
Celesteela is the Leech Seed user of the team. (Yes, I know it's rated OU.) Leech Seed is a great force switch, allows for at least one turn of heal, and does chip damage. Leftovers is to recover some health every turn. Ingrain is basically to double Leftovers' effect. Flash Cannon, again, is used as a fixed damage attack that's the strongest against opposing Pokemon, but Heavy Slam or Flamethrower can be used. Given EVs are for maximum health, and increase to Celesteela's low-ish Special Defense, and a small increase in Flash Cannon damage output. If using Heavy Slam, put the 4 Special Attack EVs into Attack.