To get the Flamerang, you need to defeat Bull, the First Boss. Afterwards, switch to the Flamerang whenever you see an enemy as you play through the game. This may be tricky considering there's an achievement for killing 200 enemies with Bites, but since you only need 100 kills with the Flamerang, i recommend using it only with enemies that are easier to kill with Boomerangs, you'll get it quickly that way and can focus on 200 Bite Kills.