I literally don't think it's possible to do anything remotely useful with Zen Mode Darmanitan. A Sub always has the defenses of the Pokemon behind it, meaning Zen Mode Darmanitan's Subs will by pretty bulky. The problem is that it's a Fire/Psychic type with base 55 speed. That leaves it with weaknesses to Dark, Ghost, Water, Ground, and Rock. Almost any attacking Pokemon in the game will carry a move of one of those types, so nothing should have any trouble doing the ~100 damage necessary to break Zen Darmanitan's Sub, even with its good defenses. And what's the payoff? Base 140 special attack (the same as Chandelure) with less speed and worse coverage, and probably less starting HP than a fully invested Shuckle. On top of that, its best boosting item (Life Orb) takes off 1/5 of its "post-transformation" HP, and that's not even factoring in sandstorm or hail stripping away your health each turn.
I think Zen Mode Darmanitan is literally one of the worst-designed Pokemon in history. It wants to be bulky, but has to lose half of its health to do so, and it's stuck with horrible typing. It wants to be a powerful special attacker, but it has atrocious speed and crappy coverage. And, of course, there's the Belly Drum factor: you COULD Belly Drum to get to Zen Mode in one turn, but then you're giving up a freaking +6 Sheer Force Darmanitan, a.k.a. "so insanely strong it doesn't even need Flare Blitz to murder everything", for...base 55 speed, a good special attack, and less attack than Crawdaunt, despite being at +6. WinstonShnozwick, I applaud you for trying, and I racked my brain to find anything Zen Mode Darmanitan could do other than "die quickly" and "be horribly outclassed", and all I did was ended up with a proof of how insanely horrible it is.