When you catch Zygarde, it knows Earthquake, Camouflage, Crunch, and Dragon Pulse, but no Land's Wrath. It's level 70, and according to its page on this database, it learns Land's Wrath at 26. How do you get Zygarde to learn Land's Wrath? Can you?
Yes, you can get Zygarde to learn Land's Wrath, but you must go to the move reminder to do it. (He/She (I don't remember) is located in Dendimille Towne in X/Y and Fallabor Town in ORAS)
Hope I helped!