This one is probably best done boosting although it could be, in theory, done with natural progression of multiple playthroughs. Essentially what you are looking for here is when a walker goes to grab you, there will be a quick QTE segment in which pressing will knock the grabbing walker off of you.
A good place to boost this would be on the Fontana mission right in front of the theater as there are throngs of walkers outside. Rallying them together, hit the first one, and then watch as they chain try to grab you. If you get good at this sequence you may be able to deflect upwards of 15 grabs per death.
Remember that these stack through deaths, restarted levels, and playthroughs.