Before You start with this keep in mide You'll be changing a filr so make a backup before You start with this. Now, go to the game file and open with notepad: singledataconfigautoexec.cfg Then add these lines to the end of the file:
Save the file. Now You must Start a new game for these to work. Your weapon ammo won't drop or your health. The enemies will also ignore you. Remove any of these codes if you wish to not have their effect.