I'd be like "Faggit...". Lol joking.
I'd ask him something to distract his attention for 1 second, I would then out of nowhere just push his arm, so the gun wasn't anywhere where it could kill me with a single shot, I don't care if it would shoot me in the leg etc as I wouldn't be dead and still have a chance.
I would then punch or elbow him in the throat, try it, see how much it hurts with a friend, this would maybe cause him to drop the gun, most likely it would anyway. Then I would just try and put the gun, throw it or whatever, as far away as us as it can go. Then basically use he stuff I know to break his arm etc etc. Put him in pain, then out of rage I'd go get a knife and hack him up into pieces slowly an painfully to teach him a lesson.