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Zen Mode is hardcoded to only work for Darmanitan. When the wielder's HP drops below 50% of its maximum, it will change forme into Zen Mode at the end of the turn. If Zen Mode is removed, or if the wielder recovers HP to over 50%, it will change back. Role Play will fail when used on the wielder, and if the wielder uses Entrainment, it will fail.
To reliably get Darmanitan's HP to 50% is very difficult, and at that level, it is easy to be KOed. The only really reliable way to do it is to use Belly Drum, which is pointless as in its Zen Mode, Darmanitan is seriously slow and has an extremely low Attack stat. As well as this, lowered Speed does not aid Zen Mode Darmanitan's sweeping capabilities, and so Darmanitan should stick with the use of Sheer Force and physical attacks. It's not really clear what Game Freak were thinking when they thought up this ability.
Found in Dream World