My first choice? Don't. That weight difference means that if he gets a grip on you, you're in much trouble.

My second choice? A weapon. Preferably a gun, but a 4″ minimum knife will do.

I'm…somewhat athletic, these days at 51. I'm pretty strong…but 100 pounds is one hell of an advantage, especially if your opponent knows how to use that advantage. From your description, he does.

Don't go aggravating apes, unless you have no choice. And then you are intent on hurting the guy. I mean hurting badly. Breaking joints, eye gouges, everything you can.