Lenny the Lyrebird is a character you meet at Lyre Lyre Pants on Fire, he's a liar who will lead you to the wrong directions of the stage, as well as one of my favorite characters from the game. For this achievement, you need to hit Lenny 15 with the Flamerang that you get after defeating Bull, the whole stage is about following around, so keep hitting him until you get the achievement.