1- draw and fire
2-draw and point, back up putting space in between us. yell something insane like
YOU $%^&*&$%^ YOU BETTER GET OUT OF HERE OR ILL BLOW YOUR BRAINS OUT!
try to scare him off to remove the danger and call the cops. if he advances fire.
3- hand on gun, maybe draw and keep pointed low or out of sight. back up and put more space in between us. Say you better not do this im armed, get out of here im calling the cops.
If they present a weapon, proceed to 1 or 2, if not, you should really have no trouble just walking away while controlling the situation and calling the police.
If they for some reason try to use their bare hands to rough up a guy with a pistol, i dont really know. Personally, both legally and ethically, i dont know how i feel about shooting. Its a tough situation, i guess learn some kung-fu.
This is all IMHO, and living in IL i have no personal experience, just my humble desk chair analysis.