Well actually, I really spent 3h on it...
Every graphics used are simple shapes, simple particle system and the built-in unity post process where I only toggle bloom effect for polish.
The code is only trigonometry and simple 2D math stuffs or simple unity methods like Rotate, DOTween libs and built-in methods.
As an example, I only design the last boss. Then, I disabled some of it parts to create the other first bosses.
This little tricks allows me to create more content with the same amount of work.
But you don't have to believe me.
I hope you enjoyed the game anyway :)