This guide is translated from Danfeng's 如何达成“固执者”999.
https://cs2bus.com/sharedfiles/filedetails/?id=2172215056
The Stubborn has been one of the hardest achievements of Pikuniku to tackle with, and it was the guide by Danfeng that worked for me. Therefore, I translated the guide under Danfeng's permission, hoping that it can help more players like me to get The Stubborn faster and easier.

When you entered the game, position yourself according to the image above, where the leftmost vertex of you and the rightmost vertex of the 2nd '0' lie on the same line.

Wait for the ball to drop onto your head.

Since the ball bounces by itself, you don't need to take any further action once you're in place.

If the ball happens to drop from a certain sweet spot, it will enter some loop and bounce between your head and the wall.

Sit back and relax!

It'll take around 3 minutes to enter the loop and around 15 minutes to reach 999. The score counter will start over from 000 once your score exceeds 999.

And there you have it, The Stubborn!

I tried this method five times and for me, it worked every time.

Your high score won't be updated if you exit the game by directly pressing Esc (or B if you're using a controller). To update the high score, make sure to finish the game by dropping the ball onto the ground.

If you grew tired waiting for the ball to drop from the sweet spot, you can try to manually bump the ball to the right before positioning yourself. The downside is that your position may be inaccurate, causing the ball to fall to the ground before you reach 999.

In the rare case where the ball fails to enter or stay in the loop, do not simply leave the game running for hours. Instead, the following tips may help:
- Double-check that your position is accurate with a ruler.
- It is also noticed that for some players the ball keeps leaving the loop after a few bounces even when the position is accurate. In this case, try changing to different resolutions as it worked for them.

This guide was inspired by the full achievement guide on Gamersky[www.gamersky.com], which mentioned that the ball can enter some "loop".

If you find this guide helpful, don't forget to visit Danfeng's original guide and leave a like/comment. :>
https://cs2bus.com/sharedfiles/filedetails/?id=2172215056
If you found any spelling/grammatical mistakes in this guide, please comment below. :D