Gonna drop my thoughts on Marshadow based on experiences. To start off, I think because of this addition, Stall is gonna become much more prominent than it already was. Classic Deo-S Hyper Offense, will die out. I think every team will be forced to use a marshadow check, especially in first couple weeks, when everyone is trying it out. The addition benefits stall. It isn't a stallbreaker, and pretty much every supportarc not weak to one of its stab walls it, along with a couple more, including Mega Sableye, bulky Ho-Oh if marshadow doesn't use Rock Tomb and probably the tip of the iceberg: Toxapex! (Marshadow can't fuck with this lol) I think the offense you will see. Will be primarily Sticky Webs, as it makes Marshadow slower and they wouldn't really care about its presence. Secondly, will be psychic terrain teams. They usually have Deoxys-Attack, and that in terrain is pretty hard to deal with in the first place. Not to mention the presence of Mewtwo, which also outspeeds. Bulky offense, with like a supportarc will most likely be the most common form now to play against marshadow, as it has tanky switchins, like xerneas. I think the viability of every arceus type which is weak to a stab of it, will lower.
As for sets, I think it seems pretty straightforward that the main set will be:
Marshadow @ Life Orb
Ability: Technician
EVs: 252 Atk / 4 Def / 252 Spe
Jolly or Naive Nature
- Close Combat
- Spectral Thief
- Shadow Sneak
- Rock Tomb or Hidden Power [Ice]
This set has probably been theorymonned by everyone before its release, and there's no one who can claim the set really. Close Combat and Spectral Thief, give marshadow crazy STAB coverage. It hits everything at least neutral, and hits with nice damage. Spectral Thief, albeit weak, can steal stat boosts. This move is huge on paper. I'd say in practice it lacks a bit. In power, utility, etc. However, it still is it's strongest Physical Ghost STAB marshadow has access to, which is why its used. Shadow Sneak is probably one of its greatest moves. It has so much utility. It OHKO's Mega Gengar, and can revenge kill stuff with quite a lot of power. Hidden Power Ice is used for Mega Salamence, Rayquaza, and Zygarde-Complete. Ice Punch is a bit weak against Mega Salamence (especially after an intimidate) and Zygarde mainly. A Technician boosted HP Ice fits this nicely. Another option which is slashed with it, is Rock tomb. This is mainly for Ho-oh, being able to OHKO it's standard bulky spread 87.5% of the time. Rock Tomb isn't just for Ho-Oh, it also has utility in having speed control, which can help you setup a Z-Geoxern for example. Life Orb is a must-have item on this pokémon I'd say, or it will really lack in power. For it's last slot, there are a bunch of other options as well. Pursuit, Toxic, Bulk Up, to name a few.
All in all, I think Marshadow will have a big impact on the offensive spectrum of the metagame. Though still lacking a bit against stall. I think it's nice, but it is flawed, and not perfect. This pokémon has to come in on a resist, or immunity. Or else it will take heaps. I can safely say Marshadow will be a late game sweeper, and it will also be able to blow holes in some balanced builds. This is pretty much what I think about it lol.