When you are low on health, go to a mushroom ring and save your game. Then, quit the game and load your saved game and you will now have full health.
Light rune: In Chapter 3, a witch and her sister will give you the rune and a spell.
Fire rune: In Chapter 4, defeat the Beguiler and his clown to get the rune.
Water rune: In Chapter 8, reunite the lost brothers to get the rune.
Dark rune: In Chapter 10, reunite the two children with their mother to get the rune.