HOW TO GET 9 HP IN CUPHEAD!! (Hearty achievement)

To have 9 hp, you have to use the double heart charm then turn on yourself for several seconds. After, for all bosses and levels in easy and normal mode you would have 6 hp 3 hp thanks to the charm. The boost stays until you finish a level. Note: I think you need to have the latest game update () and the achievement can’t be unlocked yet (probably when the dlc is released).