Let me explain it better =D
You could create a program that reads your health, it would reset it when you got hurt, but it would also keep track of how much damage you recieve. It would subtract the amount of damage you recive from whatever number you specify, and then to display it, you would have some text (like text event text.) that got memory edited, to display the number in the correct place, to prevent the text from moving or going off the screen you would nop the ASM that makes the text move.