Good job making it to the end!
You get the cutscene of the boss getting damaged when you hit it with the ball for the first time so I thought it was clear enough that this is the way to beat it. I wanted it to be simple: only one way to damage the boss and it's the same during the whole fight. The orb is not supposed to be bouncy. Not sure how you could see bounce off of the tail!
Thank you for the feedback :)