There is no way to restore health, so basically you need to minimize the number of enemy attacks (each of them inflicts 1 point of damage) and for that you may want to attack an enemy only when your power is 2 or 3 (preferably 3) and retreat when it is 1. Never enter a room with "disturbed spirit" (i.e. an unexplored room) while being chased by a "vengeful spirit".