This process incurs several continues, which have no bearing on the game's ending. At the first El Gigante fight, there is a Yellow Herb in the boss arena. Take this item and combine it with any of the Green Herbs in the same room and use it. This increases Leon's lifebar slightly.
Now let the boss kill Leon. Since the boss' room is at a map transition (the green OPEN command is a dead giveaway), this lets you continue right at the boss fight but with Leon's life increase from your previous attempt. Repeating this process will increase Leon's lifebar to maximum prematurely. Extra Yellow Herbs found later can be given to Ashley, or sold to the merchant for money