Heal yourself with a medkit 100 times.
Medkits will be scarce throughout the entirety of the game. The best option for this is to use whatever cash you have after completing the game to buy the remainder of what you need and use them all in a row while standing in some fire. You can start a new game and grab the one in the bathroom of the first room in the Prologue, use it and reload checkpoint to farm this. You can also load chapter 18 in a NG+ as well, there will be a handful lying there in the beginning.