What you could do, is have a loop on the player. It checks to see if the players life is less than 100, and stores how much damage the player has taken, then heals the player. You can subtract the damage taken variable from another custom made variable that the player has, newHealth for example, and then you can have another check to see if that newHealth variable is below zero. If it is, kill the player.
You could also instead set the players health to 50, then you can use healing items to have the player heal. Getting a custom playertype with massive amounts of life, and setting the players health to the halfway point gives you extra leeway between healing and damage.
See if you can try to implement this on your own, just post here again if you can't figure it out.