Try to make a mark of some sort to lead you back to some Medkits that you could not carry before. If you so happen to run low or out of Medkits because of a big fight you can find them again. This is useful in Interval 11 with the armies of ghosts coming after you. Just make sure that you are able to backtrack to get the Medkits you left behind. Additionally, use your Medkits at the correct time. Do not use Medkits immediately when you have about 70 to 80 health remaining. Try using a Medkit when your health drops below 30. One Medkit can heal up to about 30 to 45 points. If your health happens to drop below 40 and you are in a 1v1 or 2v1 fight, do not bother using a Medkit unless you really need to. Just run to cover while using your slow-motion abilities. Your health will regenerate slowly back to 40 if it drops below that; however, your health does not regenerate during a battle.
