You have 3 ways to do this. First is to gain many experience, so when you level up your HP will increase. Second way is to use Skill Book 'Life Up' ability. To do this you must first level it up to MAX so it can increase your HP to 800HP to your maximum HP. Third way is to buy an item from the guys before last battle, but this only plus 10.
About half way through the game you will encounter a beastman settlement where the elder wishes you to go the hall of trials to get some moss throgh the game of collecting poff nuts. once you are done with the tasks in the village keep going back to the hall of trials and play the game continuoslly. you should get the following for the amount of times you play [ i believe these are in the right order]
once: platinum feather
twice:blood moon [weapon for tio that restores health]
thrice or more: scales that double the amount of gold you get from chests, monsters and bosses.
