"During this event, you may encounter Bronzong and Lucario in 4-star and 5-star Tera Raid Battles. The Tera Types of these Bronzong and Lucario will vary, and if you catch them, they’ll make powerful teammates in the battle against Greninja with the Mightiest Mark! "
They're giving us a Fighting/Steel (with Justified potentially) and Psychic/Steel (with levitate, potentially)
Bronzong seems as miss as Gengar was, why would you ever use a pokemon weak against the boss stab?
Lucario seems a bit better but still it's a very frail mon so not something I'd want to bring.
That said, in this case I'm mostly expecting the 3 stabs + a setup move
Here's the first interesting bit: it may be Mixed. Greninja's signature move (Water Shuriken) is special, but the only actual poison attack it gets is Gunk Shot.
(Worth mentioning that if Tera Blast is picked instead of Gunk Shot and Greninja is fully special, then Blissey likely can eat everything it throws at her)
Going through its moveset, the only notable setup move it gets is Swords Dance, which would allow it to go full phisical in theory.
I do have a hunch however that the gimmick will be Double Team spam.
Depending on moveset, I think the main issue in this case will just be having something that can actually not die.
Poison is weak against Psychic and Ground, and the Greninja actual stabs hit both supereffectively, so you won't be able to just slowbro your way through most likely.
If Haze is the play (in case Double Team is the gimmick), Toxapex seems to qualify as the premiere option. Resists 2 attacks, can tank the neutral one, can easily both Haze or Clear Smog away potential buffs while being also immune to poisoning.
Water Absorb Gastrodon and Clodsire may also be serviceable (bonus, Gastrodon has Clear Smog, clod Has Yawn, Acid Spray, Haze, and both hit with supereffective stab while also having access to Mud Slap and Chilling Water)
Outside of these 2 however I struggle to see any pokemon who can hit with supereffective stab without being hit back supereffectively by one if the 3 "stabs".
I also don't quite see any reliable Stored Power spammer this time. At least, not stab ones.
Interesting but maybe memeworty options I can see:
Umbreon, Dudunsparce, Blissey (if it can tank Gunk Shot...), Klefki, and Flutter Mane.
Umbreon and Blissey don't really have a notable spatk so would need absolutely hilarious levels of buffs to produce a 1hko though, and Flutter Mane doesn't resist any of the attacks...
Assuming a 30x base HP like Cinderace...that'd mean 8550 HP, let's assume 6 Calm Minds...
+6 252 SpA Blissey Stored Power (260 BP) vs. 0 HP / 0 SpD Tera Poison Greninja: 2080-2448 - Not even close, and takes up of 84% of gunk shot. However absolutely laughs at any special attack if it's tera blast instead
+6 252 SpA Flutter Mane Stored Power (260 BP) vs. 0 HP / 0 SpD Tera Poison Greninja: 3082-3626 - Better but still not remotely close, similarly getsalmost oneshot by gunk shot but eats everything else ok
Umbreon can actually eat gunk shots almost for breakfast but hits for even less than Blissey
All in all, I don't quite see this as being another oneshot cheese. Will have to likely work out similarly to Charizard where one person or two keep up the party while the others wittle it down.
Notable: Water/Fighting resists all 3 attacks... but
If we actually had Poliwrath maybe Belly Drum stuff would have been possible, while Urshifu or Keldeo have a lot of raw power
Quaquaval doesn't exactly strike me as good for this sadly.
Edit: I just realized also Dry Skin Toxicroak resists all 3, but then again, still the problem of not exactly having any raw power. Gets EQ to hit supereffectively, I suppose, though non stab.
Edit n2:
+6 252 SpA Flutter Mane Stored Power (260 BP) vs. -6 0 HP / 0 SpD Tera Poison Greninja: 12456-14656
I think I cracked it, if it's special only. Oneshots at +5 as well, or +4 with Psychic terrain.
If it's mixed however i legitimately can't find any pokemon that has Stored Power AND doesn't just drop to one of its attacks.
Clodsire: can be immune to Water via Water Absorb, doubly resists Poison, is immune to poisoning, can boost with Curse, has STAB Earthquake, has great Special bulk, has Haze for enemy boosts. It is however weak to Ice.
Quagsire: it too has Water Absorb, resists Poison, has STAB Earthquake with slightly higher Attack, can boost with Curse, has Haze and is not weak to Ice
Gastrodon: has Storm Drain, STAB Earthquake and Earth Power, Curse, Clear Smog, Stockpile, Yawn
Toxicroak: has Dry Skin, resists Dark, resists Poison, has Swords Dance for boosting 
Tatsugiri: has Storm Drain, high Special Attack, Nasty Plot, doubly resists Water but has no useful coverage
Slowbro / 
Slowking: the Stored Power nuke could still work, with Amnesia over Iron Defense if Greninja is fully Special, but they are both weak to Dark
