>>46298572252+ Atk Choice Band Regigigas Superpower vs. 0 HP / 4 Def Zacian-Crowned: 222-262 (68.3 - 80.6%) -- guaranteed 2HKO
No possible situation where Regigigas can OHKO Zacian-C with Superpower.
252+ Atk Regigigas Earthquake vs. 0 HP / 4 Def Zacian-Crowned: 248-292 (76.3 - 89.8%) -- guaranteed 2HKO
You need a Choice Band just to KO with Earthquake, which means you can't run Scarf and thus Zacian will always outspeed.
+1 252 Atk Zacian-Crowned Close Combat vs. 252 HP / 4 Def Regigigas: 442-520 (104.2 - 122.6%) -- guaranteed OHKO
Even if you're running max HP, you still get destroyed by CC (and Regigigas would likely not be running max HP, since it benefits heavily from speed investment).
