First of all, beat the game. Then, you will be in your bed. Leave the house and go to Desert Resort. There, if you go in front of the 6 statues, I think -can't remember correctly- that Juniper (the girl) will give you a RageCandyBar. Use it in front of a statue. It will be a Darmanitan, normal form. Lower its HP with moves like Dragon Rage and Endeavour but do not kill it. Its HP will go down to red, and whenever the Darmanitan you wake up from the statue's HP goes lower than the half, it gets Zen-Mode. Catch it, and feel free to use it. Or, if Juniper doesn't give any RageCandyBar, go to Icirrus city in the Winter. There is a house where someone gives you a RageCandyBar.
But you want my opinion ? Darmanitan-Z is not worthy of a try as its low speed overwhelms his other decent stats. Use Sheer Force Darmanitan. But in-game, I believe it should be fine.
Though, this is for BW 1. As for BW 2, there are no ways, except trading, to get Darmanitan-Z, which is not worthy of that work. If you still want it, I am sure there must be people in chat room who has it, you can arrange with them a trade.
Hope I helped ;)