Tough Claws Trapper

A fairly simple set that aims to eliminate Ghost-types including Mega Gengar, Giratina, and Aegislash with Pursuit in order to support the rest of the team. Typically you'll use this alongside other physical threats like Regigigas, Mega Beedrill, Primal Groudon, or Mega Mewtwo X, all of which vastly appreciate the removal of Giratina.

Do note that the idea isn't perfect: Fur Coat Giratina, as well as variants like Poison Heal King's Shield, can escape unharmed. As a result, when building you have to be a bit more conscientious around these types of Pokemon. For instance, a set like Stealth Rock PH Regigigas, which aims to exploit the removal of specifically Magic Bounce Giratina, is a good idea. Another one is SD Photon Geyser Mega Mewtwo X, which gets stopped by specifically Prankster Giratina.

When Greninja isn't trapping Ghost-types, you want it acting as a straightforward breaker. Here, Close Combat--not Knock Off--is the more prominent move surprisingly enough, aiming to hit the variety of Fighting-weak Pokemon in the metagame as well as Imposter. Think of it like a worse Mega Mewtwo X that doesn't lose to ghosts.

In the last slot we have the logical Sunsteel Strike to hit Xerneas, but Ice Hammer is a very worthwhile alternative. The reason for this is that many Greninja structures will be weak to Zygarde-C due to it fulfilling a similar role to Giratina while being resilient against Pursuit. Thus, having an ice move in your back pocket will go a long way in that matchup.

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Bait in opposing Ghost-types with a teammate and get Greninja in there. Greninja fears U-turn from defensive Ghost-types and setup from Gengar, so switching in is far from risk-free. An Eject Button teammate can bypass the risk altogether, grabbing Greninja an opportunity at no cost.

Giratina is very bulky, with +Def variants taking 75% on average from switching-boosted Pursuit, while Knock Off will do about 62% to Griseous Orb variants and have a small chance to kill if removing an item. Thus, prioritizing hazards to take off the last bit of health can be very important, as Giratina can very easily find an opportunity to recover and try to win another Pursuit 50/50.

Neutral-nature Aegislash takes on average 58% from unboosted Pursuit; Mega Gengar takes 85%. Obviously, neither of them survive a missed prediction.

Greninja isn't a great wallbreaker outside of its Pursuit role, with only Fighting-weak Pokemon such as Registeel really being scared of it. Even something with middling physical bulk like Primal Kyogre can just comfortably take a hit and recover or pivot. The hardest non-passive counter to Greninja is Primal Groudon, which takes a mere 34% from Knock Off on average and deals around 89% with neutral nature V-create in return.

Defog on Fur Coat or Poison Heal Giratina can thwart the typical Greninja team initiatives, but it's also very passive, meaning the onus is on Giratina's team to convert the deadlock into an advantage.

Lastly, some teams have run Prankster Baton Pass on Giratina and Aegislash specifically to escape Pursuit. There really isn't much you can do about this.