The next Unrivaled Tera Raid has arrived, and this time, it's Primarina! Overall, it was pretty easy to predict how to counter it, but it turns out to have Sing, so you'll want to have an ally with Safeguard (such as Arboliva) to protect you from it. I use ArceusSteel to wall it and take it down:

Arceus @ Iron Plate
Steel Tera
Ability: Multitype
Modest, 130 HP, 128 Special Attack, 252 Special Defense
Judgement
Calm Mind
Recover (or Electric Terrain)
Acid Spray

The main thing to do is set up 3 Calm Minds so you can start to wall it. With that, you can start using Judgement, because at 85% time remaining, it resets its own stats, and at 80% time remaining, it'll reset yours. Hopefully you are still awake, so you can use another 3 Calm Minds, Recover if needed, then use Acid Spray to charge up your Tera Charge. Once you are ready, a Terastalized Judgement should do a good chunk of damage and help you take it out with east.

If you are having trouble with Sing, you can try using a few heal cheers to wake yourself up, or use Electric Terrain (over Recover) to prevent yourself from falling asleep.