Think of the Boomerang Bro as a cousin to the Hammer Bro and Fire Bro. Of course, as the name implies, the Boomerang Bro will throw boomerangs whenever you get close enough to them. Being a boomerang, these projectiles will return to them, so you not only have to worry about the initial throw, but also the return. It only takes a single hit to defeat a Boomerang Bro, with most of the normal methods working. Just remember to keep an eye out for that returning boomerang, even after defeating the enemy.