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 (1.3.2) and the achievement can't be unlocked yet (probably when the dlc is released).

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