To get infinite HP for Salmando (the red dragon that is typically associated with the protagonist's party in battle), turn him into a Master Dragon; unlike the other dragons in the game, only Salmando will never have his HP decrease on taking damage. The condition for him to be a Master Dragon is the same as the others: get 250 in each of Strength, Dexterity, Vitality, and Mind; get 100 in each elemental stat; and have less than 100 Loyalty (since Uni Uni overrides Master Dragon and is much less desirable).