>>49854963Tough question.
I think it's worth noting that electric, fighting, water, ghost, and steel types all hit Zacian for neutral damage.
If you can't get away with hitting them with fire/ground type moves, you can always use these other options to counter zacian.
It helps that all these types have been historically prominent anyways.
Without boosts though, Fire and Ground are your only ways to one shot it. Most common options for the neutral hitting types can do at least 50% or greater though, so that's worth knowing.
for instance, life orb Modest Zapdos with thunderbolt does minimum 55% to zacian (albiet with rain thunder does min 67, up to 80%)