This thing took me up to an hour and MULTIPLE failed attempts to capture. I swear, I will never strive harder to catch a single Pokemon, Legendary or otherwise. But, it proved worth it in the end. That feeling of accomplishment was like nothing else when that little shower of stars came from one of many Ultra Balls. Though I deleted my file shortly afterwards, it was a rewarding challenge to capture this ground snake...thingy.
Funny thing... I lowered Zygarde's HP, and it camouflaged itself, then I used Pikachu's Thunder Wave to Paralyze it, then I threw one ultra ball, and BAM! It took three shakes and a click with one Ultra Ball, thanks to my Capture O-Power lv.2!