My original idea to use Brute Bonnet won't work because this Pikachu has Play Rough, so I went with Gogoat instead. Pairs well with Arboliva as Grass Pelt kicks in once Arboliva's Seed Sower creates Grassy Terrain. These are the details I have:
Item: Covert Cloak
Nature: Adamant
Ability: Grass Pelt
Tera: Grass
Effort Points: 252 Attack, 144 Defense, 64 HP, 50 Sp. Defense
Sunny Day
Grassy Terrain
Horn Leech
Milk Drink
This is a set designed to function well regardless of what the teammates use, as there's no guarantee people will be using Arboliva, Meowscarada, or Lurantis (I haven't seen much discussion about Lurantis in English-speaking circles, but I noticed Japanese players prefer Lurantis over Arboliva or Meowscarada). Keep up the harsh sunlight and Grassy Terrain if no one else has set them up, Horn Leech will heal HP and, once you can Terastallize, will heal all of Gogat's HP, and Milk Drink is for if Gogoat's HP gets low and you haven't ben able to Terastallize yet. Covert Cloak is to prevent Paralysis from Thunder, and I had some awful luck with getting fully paralyzed.
Once Grassy Terrain goes up and Grass Pelt activates, Pikachu will stop using Play Rough and Iron Tail and will use only Surf and Thunder. (I might put up a video of it later.)
How do you prevent Pikachu from knocking out Meowscarada repeatedly with Play Rough? I saw people try that, only to get their Meowscaradas fainted repeatedly. We failed one raid because two people brought in Meowscarada, and they each got knocked out four times.
(Do you still get the Light Ball afterwards?)