The key to crispy skin on a branzino, or any fish, is to start with dry fish. Before cooking, pat each filet dry with a paper towel. If time allows, leave the fish in the refrigerator on top of a paper towel lines plate for an hour or so. The paper towel along with the dry atmosphere in the refrigerator will air dry the skin even more.  Another crucial step is to use a very hot pan. This is a great task for your cast iron. Be sure to preheat it well before placing your filets in the pan. Allow the fish to sit in the pan undisturbed, until it begins to turn golden brown around the edges, before you attempt to flip.