Question

How do you get Darmanitan-Z?

Answer 1

Get a rage candy bar look herein how to do that http://bulbapedia.bulbagarden.net/wiki/RageCandyBar
Then go to desert resort and in it you'll see an entrance to the underground cave thingy and 6 statues around it, go up to one and hit a. Give it a rage candy bar and it will wake up, catch it and that's a darmanitan with zen mode. When it gets less than 1\2 health it becomes zen mode.

Answer 2

Once you beat the main story of the game, go to the desert resort and Professor Juniper will give you the ragecandybar. Walk up and interact with one of the blue statues (Professor Juniper will instruct you to do this), and you will be thrown into a battle with a lvl 35 Darmanitan. Catch this darmanitan, and it's ability will be Zen Mode. In-Battle, when this darmanitan's HP is 50% or lower, it will activate Zen mode.