I'm not sure if anyone needs this, but I decided to make one anyway since I was struggling with the final boss. This video might also help people who are too lazy to learn the game mechanics to finish the game faster.

https://www.youtube.com/watch?v=VH9SChzd0NQ

Timestamps (chronological order):
02:04 - Pillar
03:31 - Labyrinth City
05:36 - Cemetery
09:22 - Cemetery II
13:21 - Forest
16:41 - press up, then left
19:11 - Forest II
23:30 - Labyrinth City II
25:13 - Pillar II
26:10 - Hill II
31:11 - Hill
36:26 - Boss
36:43 - Ending
37:06 - Broken minigame

Timestamps (stage order):
31:11 - Hill
26:10 - Hill II
05:36 - Cemetery
09:22 - Cemetery II
02:04 - Pillar
25:13 - Pillar II
03:31 - Labyrinth City
23:30 - Labyrinth City II
13:21 - Forest
16:41 - press up, then left
19:11 - Forest II
36:26 - Boss
36:43 - Ending
37:06 - Broken minigame

Tips:

• Press down arrow to cast shock (lightning). You can't cast it while you're moving, so make sure to stand still before you do that.
  
• Since you can start the game from any stage that you want, do it from the hardest stage. As for me, the hardest stage is Pillar because of the edge jumps. The rest should be easy after you know what to do.
  
• Your progress will reset if you run out of health, so make sure to collect as many hearts as you can.
  
• You must do this in one sitting. The game doesn't save any progress except for the minigame score.
  
• All stages except Boss have an extra health. Prioritize in taking it first if the heart is easy to get.

There were a lot of deaths in the video, especially on the final boss since I didn't know what to do. The thing that I did on my last health was a desperate move, which surprisingly worked. Strangely, the boss didn't die when I attacked it in my first run so I thought that you can't attack him.

Luckily, I don't have to spend more time to finish this game anymore - I might go crazy because of it. Also, no, I'm not doing it in normal mode. It's just not worth it and will take too much time to finish.

Anyway, it seems that the minigame is broken after you finish the game. This video can also serve as a proof of that.