Darmanitan demolishes a poor unsuspecting team.
Let me explain. For those not familiar Darmanitan's Zen Mode ability allows him to change form when its health is under half. This altered form has great defences, low speed and attack, and high special attack.
The idea behind this set is to belly drum, get below half with sandstorm (using belly drum alone will not activate the ability). With your increased defences you can take a hit, rest to gain full health and wake up with the chesto berry. BOOM! +6 Darmanitan.
It doesn't end there. Flame charge boosts your speed--it allowed me to outspeed the scarfed Heracross in the video. At +6 attack Darmanitan reaches 1264 attack. +1 speed with Jolly nature and max investment reaches 476 speed.
Screen support is highly recommended. I suppose baton pass could work too. Try to take out any of your opponent's priority users before brining in Darmanitan. Timing is crucial for this set. The set is quite gimmicky and you only have one try, but if it works...
Here are the evs and details:
[SET]
name: Insanity
move 1: Belly Drum
move 2: Rest
move 3: Flame Charge
move 4: Superpower / Earthquake / Rock Slide / Overheat
item: Chesto Berry
nature: Jolly
ability: Zen Mode
evs: 252 HP / 4 Atk / 252 Spe
The fourth moveslot is flexible. It mainly provides addition coverage for when Flame Charge isn't quite enough. Overheat, or another special move is an option because in it's Zen form Darmanitan still has over 200 speed with potential boosts from Flame Charge, so you can still do some damage when you are below half health.
This video is just a demonstration, and the set is just something you might want to try out if you feel like taking a risk.