UPDATE 1: doesn't seem to work every time, takes a little luck, you will know when you do it right if when you press "YB" frank just grabs around the "human". keep achievement tracker on so you can see its working.
started going for this achievement from only having around 15 kills and found a much easier method by accident, that only took about a half hour to get the remaining kills.
as zombie frank, pressing the "y" button lets out a loud "scream" from frank that, is almost an easier way than the "using LB in crowds method" to rack up your combo to 25.
and also yelling on "human" characters will stun them ( have their hand over their ears) and if you go up to them when they are stunned with your combo ready. you can keep pressing "YB" over and over again until they are no longer stunned.
[can also be done against the flamethrower exo suit guys, as you cannot actually grab them. frank will grab around them and will still count towards the achievement.]
AND THEY ALL COUNT this makes this achievement go by ALOT faster than one at a time.
TIP: MAKE SURE TO HAVE YOUR ACHIEVEMENT TRACKER ON
i made a quick video of how my "run" went. i just made it to this point in the story (about 20mins in.. then follow video, when clear reload checkpoint and go again.
around the 1:20 mark, what you can't see is my achievement tracker went up by about 8 kills alone right there.
please comment if anything is unclear and i will try to clarify. hope this helps someone.