Pikuniku
14 Achievements
1,000
4-5h
Stubborn
999
100
0.77%
How to unlock the Stubborn achievement in Pikuniku - Definitive Guide
Once you have the game, head back out of the mine, leave the beach, and take the train back to the forest. Then, head up the elevator, go just a little right, and jump up into the cabin with what looks like a Space Invaders logo on the door.
From there, you can use the cartridge on the TV to play the game. The tricky part is getting 999. One technique is to stand under the first number. If you manage to stand in the right spot, then the ball with just keep bouncing off your character and the wall repeatedly until you get the achievement. Here is a video as an example:
It will definitely take some trial and error though.
All credit to Mr Splash.
Edit - If you got the game cartridge, then finished the story before playing the mini-game, you can still select 'continue', and make your way to the forest.
In other words, you wouldn't have to start a new game.
Edit 2 - I'm going to highlight the comment from @Lockie since it seems to be working out better for some:
"Just did it then by placing the guy directly under the last digit and facing him left. The ball ended up hitting the back of his head really fast so I got to 999 in about 8 minutes."
UPDATE: Got to 189, and it dropped the ball for no apparent reason, didn't touch the controller at all...
UPDATE 2: Same technique, same spot, worked better this time...failed at 902...again, same reason, randomly dropped, didn't touch controller
This mini game requires you to bounce a ball on your head, each bounce is 1 point and you need to get to 999. If you miss the ball and it touches the ground the score resets. An easy way to get this is to position yourself, so that your nose/beak is lined up with the outside of the last number in the score. This will cause the ball to bounce off the edge of your head and the wall without any other input. It may take a bit of trying to get into the correct position, but once you get here you can just put your controller down. It will take around 10-15 minutes to reach 999.
You need to find the video game cartridge in the Old Mine. During the story, you will meet the Mother Worm. To the left of her is a jagged wall you can enter. There is a worm in here playing games on a computer. Talk to the worm to get the video game cartridge. Now head back to the forest and find the house with the pixelated video game character poster outside. Inside the house will be a gaming console that you can play the video game cartridge on.
This mini game requires you to bounce a ball on your head, each bounce is 1 point and you need to get to 999. If you miss the ball and it touches the ground the score resets. An easy way to get this is to position yourself, so that your nose/beak is lined up with the outside of the last number in the score. This will cause the ball to bounce off the edge of your head and the wall without any other input. It may take a bit of trying to get into the correct position, but once you get here you can just put your controller down. It will take around 10-15 minutes to reach 999.
See the video below for more information.