The Demon has a fairly uncommon ability which allows it to hunt immediately, regardless of player sanity. You need to die to this specific ability for it to count.
Important notes:
- It's best to play on a difficulty without any setup phase, because even the Demon ability must respect it.
- A normal hunt (by setting your starting sanity to 0%, for instance) will not count.
- A cursed hunt (caused by a cursed possession) will not count.
- A Mimic pretending to be a Demon and using its ability will count!
This achievement is entirely RNG though there is one thing you can do to improve your chances: use the Monkey Paw cursed possession. Wishing for activity will double ghost activity for a couple minutes, and this will double the chance of a Demon using its ability.
You can use custom difficulty settings to force the Monkey Paw to always spawn and help move things along in other ways, here's a basic setup I use:
Starting sanity: 100%
Sanity drain speed: 0%
Sprinting: Infinite
Player speed: 150%
Ghost speed: 150%
Roaming frequency: Low
Changing favourite room: None
Interaction amount: High
Event frequency: Low
Grace period: 0
Hunt duration: High
Evidence given: 3
Setup time: 0
Weather: Sunrise
Fuse box: On
Cursed possessions: 1, Monkey Paw
Play on Camp Woodwind. If you prefer Tanglewood that's almost as good. Using a timer or clock or something, make note of when you open the gate.
Beeline straight for the Monkey Paw (its on the far side of the camp on the Lovers Bench), and immediately wish for activity. If it works, the generator will break and the gate will lock. Walk around, try and find where the ghost is, stand near it and pray it's an angry Demon. If the time passes a minute without a hunt, just leave the investigation and start again.
I personally grab an EMF reader while the doors are opening - occasionally you'll get an EMF 5 reading while you're waiting, indicating that the ghost is not a Demon and allowing you to leave early. You could also use a Video Camera for Ghost Orbs, but this could potentially cause you to miss a Mimic-Demon.
Even with this strategy, this will likely take dozens and dozens of tries, so it might be worth focusing on other achievements first and just keeping in mind that if you get an extremely early hunt, it's worth suiciding into.