Hi, soundtrack composer here! Since you are open to hints. There is one boss each in Lacelode (lower left area), Verdowen (rightmost area), and Magridia (bottom area). It sounds like you've beaten the bosses of Verdowen and Magridia. And, while a lot of progression is locked behind destructible terrain, Lacelode has an ability in it which highlights destructible terrain, somewhere in the area that opens when you break through the bottom of its elevator room. Consider drawing a simple map of the area to help you narrow down where it might be, and you should be able to find the boss you're missing fairly easily once you have it c: (Of course, there are still several secrets that the Search Glass won't help you find, but hey, that's video games for ya!)