SD Poltergeist

Poltergeist hits Giratina and Lunala for huge damage while allowing Regigigas to be imposter-proofed unusually easily with an itemless Ghost-type teammate, usually defensive Lunala. The only remaining requirement for the set is some way around Steel-types, which is the motivating idea behind the fourth move.

Precipice Blades is the standard, offering the best 1v1 matchup into Imposter and hitting Flash Fire Zamazenta-C for good damage, but many other options can be considered. Fighting-type attacks are almost never seen on PH Regigigas due to worsening the Imposter matchup, so with the Poltergeist + itemless Ghost-type idea, running them starts making more sense. Force Palm is a little bit silly, but it does thwart fast Topsy-Turvy users like Zamazenta-C and FC Eternatus while providing a slight damage upgrade.

Jolly and Adamant natures are very clear, Careful is one that I randomly found in my builder with no explanation and couldn't resist including. Regigigas does typically take a lot of special hits, so I could definitely see it being worthwhile.

-trollkastel seal of approval

It's PH Regigigas, it beats most things from full HP, Facade 2HKOs the majority of the meta and practically nothing OHKOs you in return. Swords Dance is a double-down option that demands a defensive response because otherwise you really will just one-shot everything. The way to play Regigigas depends on whether your opponent is trying to hard counter it or whether they're trying to trade into it. Even if Regigigas is hard walled, it can still play a valuable role in drawing in passive mons to exploit with a teammate.

Regarding trades, be very careful; even if Regigigas technically beats the opposing mon 1v1, trading its HP off can come with severe long-term consequences. The reason for this is Regigigas's pure Normal typing, which is a double-edged sword: at high HP, it can indiscriminately muscle through targets while being obscenely hard to threaten, but at low HP, it can't get back in the game by brandishing a super effective STAB attack.

So against more passive play, the gameplans are easier to come up with, but against aggressive play and positioning, you need to pick your battles well and choose the right time to commit. Regigigas can serve as a team's beating heart, but it won't get far without the help of teammates to take some stress off it.

Zygarde-C is the one mon that this Regigigas hates seeing, as you lack both Glacial Lance and Spikes. Fur Coat Giratina also manages to avoid the 2HKO from +2 Poltergeist, taking on average 42% with neutral natures on both sides, though any chip or status can push this over the edge. Various Steel-types can also be problematic depending on the last move, with Fur Coat Zamazenta-C being the most annoying overall.