If you are going to Solo, I used the Following two builds. They both also do great in online random matchups.
Baxcalibur
The Special Defense builds worked best. If you add Breaking Swipe in instead of Brick Break you can tank the whole damage set. I recommend an Arboliva AI on your team to increase your healing, but there really is no wrong way to play.
In Online Play, just spam Breaking Swipes and cheer the teams Defenses once after Greninja debuffs the team while healing at convenient times.
Annihilape
I used Screech at first, and then I switched to Rage Fist. Terastalizing is a bit risky, as it will make Night Slash dangerous.
In online play, it is mostly the same, but Terastalizing is safer (just activate a Cheer Defense after the first Debuff).
Just a note, the Greninja uses Double Team three times throughout the battle (as well as Toxic Spikes once at the beginning). It also removes it's own Debuffs twice, and it debuffs your team. If you want to see the timing for these actions, I recommend watching this.
Hands Solo
Note, I do not recommend that build unless you are very good at Solo raids. The video is just an easy way to see when Greninja activates its scheduled debuffs. Note, someone said in another answer that these are turn based. They are timer based (and this includes if it is reduced when teammates faint) and HP based (which is why you may see several attacks and buffs go off at once after someone does a large attack that doesn't OHKO the raid Pokémon). If you do want to try the Iron Hands Challenge, the held Item was a Wide Lens, and the EVs were Sp. Def and Atk on an Adamant Nature. In online play, your teammates may attack you if you use an Iron Hands.
Finally, if you plan on supporting a team, I recommend the following build:
Clodsire @ Leftovers
Ability: Water Absorb
Tera Type: Ground
EVs: 252 HP / 252 Def / 6 SpD
Relaxed Nature
- Mist
- Acid Spray
- Chilling Water
- Mud-Slap
When the battle starts, spam A as fast as you can to make sure you get Mist off before your allies. This will eliminate Greninja's double team.
Afterwards, a Cheer Defense helps, and Mud Slap is a comically reliable against Greninja. Use 3 of them, and its already low accuracy moves will fizzle. Switch to Acid Spray (if the team has Special Attackers building up) and Chilling Water (if you want Greninja to use Special Moves more often, but watch out for Ice Beam). When Gren wipes away its own debuffs, just repeat that cycle. Do not use Mist again when Gren Double Teams later in the battle if you have Slowbro on your team (or anything else that set up buffs).
Watch for teammates in need of healing, as you can safely use Cheering Heal Up at any time.