Many Options:
Before reading this, you must take into consideration that Zacian-Crowned has made Ubers highly centralized around it, and Ubers will stay like this until Pokemon Home, where more legendaries will come into play.
Quagsire:
Unaware makes Zacian's intrepid sword and Zamazenta's dauntless shield do nothing, lowering their offensive/defensive capabilities. If you use a defensive set with unaware and use earthquake while recovering and stockpiling, you can KO zacian, at the very least.
Galarian-Corsola:
A bit of a shaky answer, but nonetheless good. Using will-o-wisp and strength sap will make zacian's attack much lower. Couple that with night shade/hex and you might win!
Rotom-Heat:
Using Overheat, you can defeat Zacian, as it is an easy switch-in as long as you pack choice scarf.
Arena Trap Dugtrio:
With arena trap and choice scarf, you can outspeed Zacian with it having no hopes of escaping, and KO it with Earthquake.
Hippowdon:
Due to great physical bulk, it can withstand a blow from Zacian and then KO it with earthquake.
Revenge Killers:
Ditto, when Zacian isn't under substitute, can imposter it and use its strength for its own uses, including using fire fang.
Excadrill, when under sand when paired up with tyranitar, can blow through it with earthquakes
For Zamazenta
Again, using unaware quagsire can work, but for Zamazenta, just remember you're fighting a defensive core with offensive capabilities. Using Zacian against Zamazenta works, as well as using Eternatus and flamethrower. It's kind of like Snorlax in Gen 2 but with much better capabilities with high defense.
These sets: Excadrill, Ditto, Hippowdon, Quagsire, Eternatus with Flamethrower, Dugtrio, Corsola, and Quagsire all work well. It's just preference and you're experience with them on smogon or in-game.
