I also study martial arts. What he did to me was rude, piggish, and totally inappropriate, but did not merit the type of response you mention-- especially since he's an old man, which does not excuse his behavior but would give me pause before doing anything to him that might break his possibly fragile bones. I suspect he trades on people's reluctance to strike an elderly person, which is obnoxious of him, but I would still feel that reluctance.
If he'd grabbed my breast, that would have been a more serious assault, and I would see a takedown or a knee to the groin as an appropriate response, no matter how old he is.
That being said, the burden should not be on women to have to defend themselves after they've already been groped. If Harlan Ellison cannot control himself, he should not be invited to conventions.