I fully expect Calm Mind as the scripted / setup move.
Reflect or Light Screen are a option as well, but seeing as Meganium just had these, I don't think it'll be the case.
The odd Amnesia is a maybe, I can see it, but more likely to have Calm Mind. Could have both though.
The other various options can be Misty Terrain (unlikely, I don't see any specific synergy), Encore (possible, but doubt it), Baby Doll Eyes (I can see this in the regular moveset), or a scripted Icy Wind (this is somewhat likely expecially given Primarina's low speed).
Do note that a scripted Sparkling Aria while having a different water stab that uses Liquid Voice is a possibility.
As for offensive moves... Water / Moonblast / Psychic feels all but guaranteed. Energy Ball or Ice Beam could round up the 4th slot.
Psychic Noise as water stab would actually be terrifying due to the healing negation, so I really hope they don't go for that one.
All in all, I think we pull out our steel types. Primarina doesn't really have anything to hit steel supereffectively.
H-Goodra stands out as a steel with access to Acid Spray who isnt weak to Moonblast and resists the other stab..
Empoleon and Dialga obviously stand out as the other two steels that resist/neutral both stabs and are neutral to or resist any coverage primarina could sport.
That said, I'm looking at Solgaleo of all things. It's one of the few good phisical steels, very strong steel stab, has recovery in Morning Sun, immune to stat drops (in case it's running debuffs), and in virtue of being a phisical attacker it doesn't care of the Calm Mind stacking.
Scizor (and well... Arceus if you really want) also stands out as a Steel type that can use Struggle Bug to neuter any Calm Mind boost as the Clear Smog users are all either bad pokemon, weak to stabs, or don't resist Moonblast. (Well, unless it lacks psychic coverage but it'd be... super free then)
Of course Gastrodon is tecnically a water immunity with recovery that has Clear Smog, but may need some shenenigans to actually facetank the Moonblasts (and ofc, folds to a potential Energy Ball coverage).