Alright I'm back from a break and here we go with dealing with your team.
Your team is very solid for laddering purposes, with Kyogre + Marshadow forming a unwallable offensive core. However, this team has a few weaknesses, notably Zekrom, SubSeed Calyrex-S and Zygarde.
Marshadow @ Life Orb
Ability: Technician
EVs: 252 Atk / 4 Def / 252 Spe
Adamant --> Jolly Nature
- Poltergeist --> Spectral Thief
- Close Combat
- Shadow Sneak
- Ice Punch / Bulk Up
With almost every Eternatus running 350+ Spe at the moment, Adamant seems a bad option on Marshadow. Poltergeist can also be forgone for Spectral Thief, which makes it easier to deal with Coil Zygarde, as Ice Punch won't hurt it much. It also does not get walled by Sitrus Berry Eternatus or itemless Toxapex, Xerneas, etc. which adds to another of its benefits.
Since you don't need Ice Punch anymore, Bulk Up is a nice way to become more threatening as it also outdamages Eternatus' Recover so that they can't stall you forever.
Necrozma-Dusk-Mane @ Rocky Helmet
Ability: Prism Armor
EVs: 252 HP / 252 Def / 4 SpD
Impish Nature
- Stealth Rock
- Sunsteel Strike
- Thunder Wave --> Earthquake
- Morning Sun
The Zekrom match-up looks really bad for this team. The only thing that can survive a +1 Bolt Strike / Dragon Claw is Necrozma-DM, but without Thunder Wave it's just easy to set-up Substitute behind it (Sunsteel Strike does not break a Sub in 1 hit) and avoid getting revenge killed from Ditto.
To solve this, you have to sacrifice one slot of Thunder Wave for Earthquake. Now with Ditto + EQuake NDM, your matchup vs Zekrom is totally solved.
Yveltal @ Heavy-Duty Boots
Ability: Dark Aura
Shiny: Yes
EVs: 252 HP / 32 Atk / 224 SpD
Careful --> Adamant Nature
- Foul Play
- U-turn
- Defog
- Roost
If your Yveltal doesn't have 32+ Atk EVs, then it can't break Calyrex-S' Substitute with a U-turn. If this happens, Calyrex-S can just set-up Leech Seed, abuse Nasty Plot and outdamage Roost's recovery. However, considering you have Marshadow and Ditto, it's not the worst mu, still investing a bit into Atk is never a bad idea.
Kyogre and Eternatus are alright, though I would go Modest Nature for the damage roles on Blissey.
- 252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 4 HP / 252+ SpD Blissey: 262-309 (40.1 - 47.3%) -- 47.7% chance to 2HKO after Stealth Rock
- 252 SpA Choice Specs Kyogre Water Spout (150 BP) vs. 4 HP / 252+ SpD Blissey: 238-282 (36.5 - 43.2%) -- guaranteed 3HKO after Stealth Rock
Though it has only 50%~ chance of 2HKOing after Stealth Rock damage, Blissey is never at 100% hp really. Many times, it has come into Eternatus' moves then pivot out with Teleport, for example.
Another thing I would go on Eternatus would be Sitrus Berry or Heavy-Duty Boots over Black Sludge. Basically, what Sitrus Berry does is that it makes you immune to Poltergeist after the item gets consumed. Heavy-Duty Boots on the other hand help vs Sticky Web teams also hazard stack teams.
Final Team
Hope this helps!
