I would recommend adding more bubble mechanics to focus more on the theme, like maybe every time you free a prisoner, you get their bubble which you can use as ammo against the evil guy.
Oh, so like, during the final battle, right? The bubbles are too big to work as ammo, so they could be used more like a "vulnerability effect" on him, like, his weapons are weakened when in it, or he is fully weakened, with no ability to do harm, of course, this will temporary (something like 20 seconds), and the bubbles are limited, thanks for the idea! I'll credit you for the idea!