Marshadow @ Marshadium Z
Jolly nature
252 Atk, 252 Speed, 4 Def
-Bulk Up
-Spectral Thief
-Drain Punch
-Rock Tomb / Poison Jab
Bulk Up with Drain Punch should attempt to get some sort of bulk in this little thing and actually hit hard. Spectral Thief handles stat boosters and is your strongest attack, especially with the Z-move. Rock Tomb gets a Technician boost and OHKO's Ho-Oh, who otherwise completely walks over you kills with Brave Bird. It's too bad you don't get Earthquake, Primal Groudon just overpowers you completely, and to a lesser extent, so does Zekrom. Xerneas is also troublesome, it only has a chance to get 2KOd by Poison Jab and easily KOs with Moonblast. And that's hard-counters, there's also stuff that can revenge kill you. Mewtwo outruns you and kills with Psystrike. Deoxys also outruns and can use Psycho Boost or a similar move for a KO. Lugia doesn't like taking Soul-Stealing 7-star Strike, which KOs if Multiscale is broken, but it can fight back with Aeroblast for huge damage. Lugia can also set up Reflect and then stall it out with Recover.
One the good sides, Marshadow OHKOs Solgaleo with Soul-Stealing 7-star Strike, and if Solgaleo tried to use Z-Splash while you switched in you don't even need the Z-move, you can then go to +3 immediately with Spectral Thief and KO at the same time, then try to go for a sweep. Be warned however, if Solgaleo goes for Zen Headbutt instead you're gone. Marshadow can also check Coil Zygarde by stealing its Coil boosts with Spectral Thief. Marshadow has an amazing type combination which gives it perfect neutral coverage with its STAB moves, although it unfortunately also gives it several common weaknesses like Flying, Ghost, and Psychic. It's fairly difficult to switch in on Marshadow, a lot of stuff fears a 2KO if they switch in on the wrong move. Drain Punch 2KOs Normal Arceus for instance, and you outspeed (Arceus can't use Extreme Speed against Marshadow). Drain Punch also 2KOs Dialga, although Dialga has a chance to OHKO with Draco Meteor even without an item boost. Palkia has a chance of getting KOd by Soul-Stealing 7-star Strike and a chance of a 2KO from Spectral Thief, although if it carries an item like Life Orb it can OHKO back with Hydro Pump or Spacial Rend. Lunala can't switch in on Spectral Thief, but if Lunala holds a Scarf it can revenge kill it or switch in on a Fighting move.
This is all still theoretical, the thing isn't even out. I'm wondering how it'll do.
It has some other options. Shadow Sneak gets a Technician boost and can be used as priority to beat Scarf Lunala, or destroy Deoxys. It can also 2KO Mewtwo. Close Combat hits significantly harder than Drain Punch and is a viable alternative. Force Palm is a bit stronger than Drain Punch too and can cause paralysis, which is also a psychological barrier for Mewtwo and Deoxys to switch into your fighting move.