If you're doing it solo, the Slowbro from the Cinderace raid wrecks Greninja ASAP

Slowbro. Level 100. Modest. Max EV HP. Max EV Sp ATK. Shell Bel.

• Stored Power
• Iron Defense
• Nasty Plot
• Psychic

Raid Greninja is a SPECIAL attacker. Its only physical move is Night Slash and it is really copium. My Slowbro has no Def EVs and it can easily tank them all after just 1 Iron Defense.

The strategy is more or less the same as with Cinderace: set up Iron Defense first, then 3 Nasty Plots. If Greninja lowers your buffs, go straight for Psychic. IF not, Stored Power.

This strategy is very consistent and doesn't rely on AI teammates doing anything although having an AI Intimidate Arcanine helps you to tank more Night Slashes.

You're welcome : )

EDIT:

Alright, here is a reworked strategy. 100% consistency unless RNG screws you over in the last part and makes you miss Stored Power. Make sure to use PP Ups beforehand!!!!

Turn 1 and 2: Iron Defense

Turn 3: Slack Off

Then, Greninja will debuff you. THIS IS THE ONLY TIME HE WILL DEBUFF YOU. At least on my 20+ runs.

Turn 4-9: Iron Defense and Nasty Plot. Do 1 Slack Off if Greninja crits you.

Turn 10 onwards: Spam Stored Power.

2 Stored Powers are enough to break his shield. And the 3rd one will kill it.

DO NOT TERA.

Yes, as crazy as it sounds, the best strategy is to never Tera at all since Hydro Pump WILL kill you if Slowbro is pure Psychic.