Use the puffer fish to pop bubbles behind the obstacles - especially at the bottom. Once I got to the bottom of the level, I spent most of my time blowing up puffer fish. They popped bubbles next to the treasures (behind the obstacles) and finished the level for me.

One you open up the area with the puffer fish, get rid of one of the corner obstacles, get rid of one of the puffer fish with a vertical combo, and then drop fat bubbles into the puffer fish area to blow them up. I got lucky and only needed one bubble to pop in there. The puffer fish took care of the rest after that.