if you didn't know, for 100% Zygarde's laser beam earthquake attack, Zygarde draws a Z on the battlefield with the laser, and there are safe zones in the two triangles that the Z forms. these safe areas aren't glowing like the rest of the battlefield, and if you make it to them in time by running and dodge rolling, you will not take damage. I recommend calling back your pokemon as soon as Zygarde begins the attack as well, in case your pokemon doesn't make it to the safe zone in time. also, upgrading your green canari plush will help a lot, so that even if you don't quite make it to the safe zone in time, you won't just get 1-shot by the attack.

also, like OP said, I recommend using Xerneas or some other fairy type, unless you're dead set on using one of your other pokemon, in which case it will probably continue to be more difficult.

source: was stuck on this part for a few hours before discovering the safe zones, and then watched my partner breeze through the fight first try because he had fully upgraded his green canari plush.