Zacian-C is one of the top threats in Anything Goes, boasting an insanely high Attack stat, nearly unrivaled Speed, a brilliant typing and ability, and access to a plethora of coverage options that make it extremely difficult to handle offensively and defensively. Behemoth Blade is the preferred STAB move, since it's stronger and 100% accurate and deals more damage to Dynamaxed Pokemon than Play Rough. Wild Charge is essential coverage for targets like defensive Ho-Oh, Arceus-Water, and Primal Kyogre that resist Behemoth Blade. Close Combat destroys Ferrothorn and heavily dents Necrozma-DM and Primal Groudon, two of Zacian-C's best switch-ins. Swords Dance allows Zacian-C to sweep once the opposition is weakened and revenge killers like Ditto have been eliminated, while also forcing the Dynamax from Necrozma-DM in order to beat it. Assurance along with Close Combat takes out Necrozma-DM on the switch after Stealth Rock damage. Crunch is an option over Assurance in order to still pressure Necrozma-DM when entry hazards are not present. Although Crunch hits said foe for the same damage as Close Combat, it has a chance of a Defense drop and doesn't lower Zacian-C's own defenses, which can be vital in preventing Necrozma-DM from reliably switching in to Zacian-C multiple times. It also targets defensive Lunala harder than Assurance and lets Zacian-C avoid a mind game with Ultra Necrozma. Substitute may be used to ease prediction against common counterplay to Zacian-C, and works particularly well due to Zacian-C's tendency to force a plethora of switches. Notably, it also shields Zacian-C from Ditto. Protect is useful for scouting Ditto and stalling out foes' Dynamax turns. Play Rough can be used in the fourth moveslot to break through defensive Zygarde-C, which checks Zacian-C otherwise. A Jolly nature is preferred with Swords Dance, as it can be used to Speed tie with opposing Zacian-C and outspeed Mega Mewtwo Y and +1 Necrozma-DM. Otherwise, an Adamant nature is preferred for the extra damage output, which notably allows Zacian-C to 2HKO defensive Arceus-Ground after little to no prior chip damage, and to put more pressure on Necrozma-DM.
He counters so many Pokemon in this format that can't be explained. Let's see how good it works:
Mega-Rayquaza: Only Scarfed ones threaten you, else he gets OHKOed by Behemoth Blade after stealth Rock damage.
Necrozma Dusk-Mane: He's one of the best counter to Zacian. Most people run Close Combat over Crunch or Assurance since they don't risk activating Weakness Policy. If its Ultra, then you are sure it won't come in a switch-in. They use Zacian as a bait since Close Combat puts dent, but it doesn't Three OHKO it. This one's a exception to Zacian's power.
Xerneas: OHKOed by Behemoth Blade. He checks you only if it successfully use Geomancy, then Max Flare.
Yveltal: Same, KOed by Play Rough. Though if it gets Max Airstream and has Heat Wave, he checks you. Apart from that, like M-Ray, KOed by B-Blade after Stealth Rocks damage.
Ho-Oh: Gone after Wild Charge.
Zygarde: Fully invested may survive a Play Rough, though he can't KO you.
Extreme Killer(loser,lol):Gone after Close Combat unless Dynamaxed, though it will get revenge killed.
Eternatus: TwoOHKOed by B-Blade, its Flamethrower can't OHKO you.
Ditto: Good check, but if Substitute, then gone.
Calyrex-Shadow: Life Orb one does 93% damage to you, but since it's not Sashed, destroyed by B-Blade.
--------Checks and counters--------
Arceus-Water: Survives Wild Charge, then can land a Will-O-Wisp.
Necrozma Dusk Mane: Best switchin, especially the defensive one.
Primal Groudon: Two OHKOed by Close Combat, but beats it easily.
Lugia: Thunder Wave one can stall PP while healing by Roost.