Just went up against Zygarde for the first time... With "Land's Wrath" and "Thousand Arrows", it rapestomped us so hard we didn't even get it down to yellow HP before "vIcToR wAs bLoWn oUt oF tHe dEn!!!1!"
Just found this on TV-Tropes' YMMV page for Sword & Shield, and it sums the issue up perfectly (except for that that, again, I didn't even last long enough to trigger Power Construct):
"From Dynamax Adventures, Zygarde is a nightmare to fight, even compared to most of the other legendary Pokémon you encounter. To start, you are most likely worn down from the prior fights with other Dynamaxed Pokémon in the dens, along with having a rental Pokémon (or a Pokémon you caught from a previous Raid). But that's not what the problem is. Once Zygarde's HP drops at or below 50%, its Power Construct ability will change it into its Complete Form and effectively replenish its health. Not only did it heal, but it also gained massive defensive boost. If you or your allies, AI or not, got a poor draw with the Pokémon available, you're in for an incredibly bad time. In addition to all of this, Zygarde has access to two party-wide attacks in Thousand Arrows and Land's Wrath, which can make short work of any party not prepared to deal with Ground-type attacks. Absolutely do this fight with friends if you can (to avoid any bad move choices from AI allies) and make sure to have at least one Ice or Fairy type on your team as well as access to Wide Guard, because while Zygarde's game never got made, Game Freak clearly want you to suffer for it."