Solgaleo @ Leftovers
Ability: Full Metal Body
EVs: 252 HP / 4 Def / 252 SpD
Careful Nature
- Sunsteel Strike
- Morning Sun
- Roar
- Toxic
This is really similar to what Slurmz posted earlier. This might look gimmicky at first, but Solgaleo's typing lets it tank some hit from geomancy xerneas and it can phaze it out. Its also conveniently immune to toxic so it can be very irritating for some support arceus to deal with. Honestly even Calm Mind Arceus that don't run refresh or aren't ghost / dark type will be slightly annoyed by it, but those aren't really popular now so...
Its slightly annoyed by Mega Gengar, but Sun steel strike will KO it after it takes two rounds of stealth rock damage. If Gengar switches in before its mega-evolved you can KO it as it comes in with just 1 round of stealth rock damage.
This thing is deceptively mediocre honestly, but it feels like it has some niche. It could be a potent special wall but its unable to check lunala and dark arceus :/. Its still not entirely outclassed by something like Jirachi because it gets Roar.
Tyranitar @ Chople Berry
Ability: Sand Stream
EVs: 252 HP / 4 Def / 252 SpD
Careful Nature
- Pursuit
- Thunder Wave / Toxic
- Stealth Rock
- Rock Slide
This is even more gimmicky, but that doesn't matter! Tyranitar should be able to pursuit trap mega gengar somewhat nicely (given hypnosis misses or you get many focus blast misses). It gets Stealth Rocks, which means you don't have to toss it on primal donner and can use another of its sets instead. Toxic means you can punish Primal Donner if they switch into you as well as support arceus that want to take away your rocks, but Thunder Wave means you can go for the cheeky paraflinch shenanigans. It should be noted that if this is attempted, the paralysis nerf going to 1/2 speed will prevent it from outspeeding things with more then 316 speed, which unfortunately has Arceus...
You can also completely forgo rock slide for something like Crunch, but Rock Slide makes you annoy Yveltal outside of status. The biggest problem is actually finding things to switch in on other then mega gar to get rocks up on :/.
Credits to whitelion for inspiring the ttar set~.
Lunala @ Psychium Z
Ability: Shadow Shield
EVs: 252 SpA / 4 SpD / 252 Spe
Modest Nature
IVs: 0 Atk
- Hypnosis
- Moongeist Beam
- Focus Blast / Psychic / Moonblast / Calm Mind
- Moonblast / Psychic / Calm Mind
So Lunala gets Hypnosis! Z-hypnosis will speed it up and is probably the only way you'll stop yveltal from revenge killing you with Sucker Punch. Its best to save hypnosis, in my opinion, until yveltal shows up, especially since it can be really hard to pin down if you have trouble getting rocks up... Alternatively you can use your crystal on Psychic to punch through something in a clutch situation, although the only thing that really comes to mind that it hits is Primal Groudon / some Arceus neutrally.
For what its worth though, the attack isn't weak:
252+ SpA Lunala Psychic vs. 152 HP / 44 SpD Groudon-Primal: 339-400 (89.4 - 105.5%) -- 37.5% chance to OHKO
(this has the psychium crystal used)
So in general having a very strong attack you can pull out is a nice luxury I guess. Focus Blast / Moonblast are coverage options. Moonblast is nice because it actually lets you KO yveltal after you put it to sleep. Focus Blast lands a strong hit on Arceus-normal and tyranitar, but those are the only notable targets. You can run both coverage options but then you lose psychic :(. You can also choose one coverage option and Calm Mind to I guess have the easiest sweep. If hypnosis hits you do get a free turn most of the time after all...
I forgot who but some guy in the ubers chat room came up with this~.