Here comes the ultimate evil set...
Yveltal @ Leftovers
Ability: Dark Aura
EVs: 252 HP / 252 SpD / 4 Spe
Calm Nature
IVs: 0 Atk
- Substitute
-
Torment
- Toxic
- Roost
If you have never played the cartridge games, you probably don't even know what Torment does, and I don't blame you because it's pretty much useless; it prevents the opposing pokemon from using the same move twice in a row, which seems really underwhelming at first. However, this effect lasts until the opposing pokemon is switched out, which allows you to beat most CM arceus formes and PDon in theory :
1. You tank judgment/fire punch/lava plume and torment the opposing mon
2. Since most CM ceuses and Pdons (mostly defensive ones, offensive Pdons might have both a fire and a rock move) run only one move that can really hurt Yveltal, you are free to Sub/Toxic on this turn, depending on whether or not the opp's main move 2HKoes you
3. Roost/Toxic depending on your health while the opp kills your sub/you tank another hit
4. Repeat ad infinitum
This process is demonstrated
here (thanks to BananaLaddersPS for the help testing this set), and although Yveltal would eventually lose the 1v1 because this arceus carries refresh, you can still beat the opposing arceus by switching in an offensive threat on a turn it can't use Judgment after whittling it down a little.
Here it is beating Fairy-Ceus as well. Finally, while you can't toxic Steel-Ceus, you can at least pp stall judgment, thus rendering it much easier to deal with for the rest of your team.
You also beat a bunch of other mons like Marshadow, as well as other Yveltas and Mewtwo if they lack taunt. This also can shut down any scarf/band/specs because of the way torment works, allowing you to freely roost after taking the first hit while your opp desperately struggles in face of this amazing set.
The EVs allow you to tank Judgment from cm-ceuses even at +2 (+1 for Fairy-ceus), ensuring that you beat these in a duel in most situations, and water spout from POgre or DAscent from MRay in case you need to toxic/torment them in a pinch. You could theoretically run the same set with full Def instead of SpDef to beat different mons (SD ceuses, ddance zygarde, etc...) depending on what your team struggles with.