Lenny the Lyrebird is the character you're after in this achievement. You need to be on the level "Lyre, Lyre Pants On Fire." To unlock this level, you will first have to unlock the Frostyrang and use it to clear the fire on the right side of Rainbow Cliffs. Lenny is the quest character you meet at the very start. Simply equip your Flamerang and hit him 15 times.