For these I'm assuming Greninja will be running Hydro Pump/Dark Pulse/Gunk Shot/Blizzard with maybe Tera Blast and Rain Dance as additional moves. Greninja has ton of other moves it can learn (Acrobatics, Low Kick, Grass Knot, Brick Break) that could mess these Pokémon up pretty badly, and while I personally don't see those as likely, build these sets at your own risk. Roughly ordered by my opinion of ease of use/viability, with the attacking sets all above the support sets
Annihilape @ Choice Band / Spell Tag
Tera Type: Ghost
EVs: 252 HP / 252 Atk / 4 Sp. Def
Adamant Nature
- Screech
- Rage Fist
- Bulk Up / Helping Hand
- Sunny Day
Screech em down and go to town. Choice Band is the strongest item but also the most restrictive. Without it, you can run Bulk Up for additional oomph, Helping Hand to be a nice person, or Sunny Day to tank Hydro Pumps better
Perrserker @ Metal Coat / Wide Lens
Ability: Steely Spirit
Tera Type: Steel
EVs: 252 HP / 252 Atk / 4 Sp. Def
Adamant Nature
- Screech
- Iron Head
- Swords Dance
- Helping Hand
Works best as a set of three, with someone else providing Light Screen and/or Sunny Day, plus heal cheers. One Screeches twice, one Screeches then Helping Hands, one SDs into Iron Head. Should OHKO if coordinated right, but even if that fails, the cats are still solid
Magnezone @ Leftovers / Light Clay
Ability: Analytic
Tera Type: Steel
EVs: 252 HP / 252 Sp. Atk / 4 Sp. Def
Calm Nature
- Metal Sound
- Light Screen
- Flash Cannon
- Sunny Day / Helping Hand / Eerie Impulse
Magnezone's special bulk isn't as great as its physical bulk, but it barely avoids getting 2HKO'd by Hydro Pump with these EVs, and can set up Light Screen and Metal Sound. Doesn't have a way to boost its SpA, but can still do some damage with Analytic Flash Cannons when Greninja's SpD is down
Gastrodon @ Ability Shield / Covert Cloak
Ability: Storm Drain
Tera Type: Ground
EVs: 248 HP / 20 Def / 240 Sp. Def
Calm Nature
- Mud-Slap
- Recover
- Helping Hand
- Earth Power
Storm Drain redirecting Hydro Pumps from teammates is its main selling point. Resistance to Poison and good special bulk means it'll stick around, but unfortunately, its support movepool is lacking, so it'll mostly be lowering accuracy and throwing out Helping Hands. In the off chance Greninja uses Swords Dance, run Clear Smog over Mud-Slap
The EV spread given makes Gastrodon take slightly more damage from Gunk Shot than from Dark Pulse from a +SpA/~Atk Greninja, which is good because Gunk Shot misses more frequently and won't flinch, but bad because it might poison. If you're concerned about poison, shift more EVs from Sp. Def to Def
Umbreon @ Light Clay
Tera Type: Dark
EVs: 252 HP / 4 Sp. Atk / 252 Sp. Def
Calm Nature
- Light Screen
- Helping Hand / Screech / Fake Tears
- Sunny Day / Snarl
- Moonlight
Umbreon gets a ton of useful support options, so it's very flexible for raids. It won't do much damage without Foul Play, but it can cut Greninja's damage output and strengthen its allies. Corviknight can't learn Snarl or Helping Hand but otherwise does the same thing, running Roost over Moonlight
Tinkaton @ Light Clay
Tera Type: Steel
EVs: 252 HP / 4 Atk / 252 Sp. Def
Careful Nature
- Skitter Smack
- Light Screen
- Helping Hand
- Gigaton Hammer
Despite its huge hammer, Tinkaton's best stat is its Special Defense, so the above set is a support set. You can opt for Swords Dance with a more offensive stat spread if you'd like to use this with a Perrserker trio, but I think you're better off on support.