This will be much easier obtaining this on casual difficulty. Even though you will stay a fair enough of damage, you will still be able to make it through the missions with your remaining health. If you are very low on health, quick die or restart last checkpoint. You will come back with full health. If you do use a med kit, remember to just use it once within that mission. There should actually be no reason for you to ever use a med kit on casual, unless you don't use stealth and just run in to battle with your pistol. Take your time with your shots and you will get through all missions without needing to use a med kit.