Cities: Skylines II

I managed to complete this thanks to a guide on Steam.
Even with the guide I struggled to get to this so I added some tips.

You want a city with at least 20k population however more is better. Your population will fall fast with this method so you might not reach 25% happiness with 1000 people left otherwise.
If you're trying to go for this on a city with a mail and internet service, disable those first. Wait for the happiness from those services to go down to +2/+3 or lower.
Now pause the game and put your water supply in polluted water. Unpause the game and allow the water to get polluted a bit until happiness from Polluted Water reaches about -10 to -14.
Pause the game again and do the following:

- Bulldoze the sewage
- Raise all taxes to 30%
- Increase all service fees to 200%
- Bulldoze police stations / hospitals / schools / parks and all other public services
- Dezone all industrial and commercial areas which will cause massive unemployment
- Save your game here in case it fails on the first try
- Unpause and set speed to x3

I managed to get it the first try after doing all of the above. If you do fail just reload your save and try again or allow happiness to go even lower. Oddly enough my mail service reliability happiness went up during all this, presumably because less people in my city meant less mail.

Preparation: I saved a version of my existing city to allow me to earn some achievements without disturbing my main game. You will have to play the game to the point 1,000 people have joined your city.

Execution: Remove the basic services that humans need and charge them for it:

• Increase the taxes to maximum for all areas (residential, industrial, commercial);

• Bulldoze any service buildings (hospitals, police/fire stations, schools etc.);

• Bulldoze/disconnect essential facilities - electricity, sewage, water; and

• Bulldoze public transport, parks, internet towers, post office, parking etc.

Set the game speed to x3 (fastest) and watch your happiness meter come down.

If the above doesn't work (good for you, your people love you), keep removing services and roads until it does. I didn't need to but you could enforce city policies to reduce peoples' free time, build industry/polluting buildings near your residential areas, bulldoze roads to stop transport and people from leaving the city.