This guide is right for you if:

• you are really annoyed trying to get that "Enthusiast" achievement;
  
• you can't feel the rhytm;
  
• for some reason you just can't beat the song for 100%.

This guide is NOT for you, if:

• you are a TRUE ACHIEVEMENT HUNTER;
  
• you hate cheats, cheaters, game-modifications etc.

You have to download an open-source app called Actiona.
Official site: https://actiona.tools/
Official GitHub: https://github.com/Jmgr/actiona

Download Script from one of these links:
Yandex Disk: https://disk.yandex.ru/d/o0Yn0a-Nm6H9yA
Google Drive: https://drive.google.com/file/d/1n_HRzqCDN_b-_JG_0-Y9R1NiykJZStEA/

Start the game. Start the Karaoke mode. Enable "Easy mode" and "ASD mode".
Minimize the game (just use Win+D).

Double click on downloaded script (or Open script file manually from the Actiona menu "File").
Now press the red gear button to start the script.


If you see this small window in your top-left corner, it means, that the script is running.


Get back to the game (maximize game window). Start the song "Lost demon".

The hardest part:
Press SPACE button just in the moment you have to press the first karaoke button. If you see "GREAT" for the first note, the song will be completed automatically.

It's okay, if you missed. Just press "Ctrl+Alt+Q" to stop the script. Exit from the song and start over. Don't forget to start the script again.

If you do not trust scripts from the Internet, you can try to make your own script in any other macros-app (like AutoHotkey).
I use "Space" button to manually syns the script and the song. You can use any other button.
Pay attention to the line 035, there's non-typical interval 3430.