NOTE: I think it's best to do this with the Critical Combo Strikes upgrade, once you get to a certain point early in your combo each hit in that combo will count as x2 in your combo meter. Did I mention the word combo?
This achievement is really about 60% luck and 40% skill. You gotta get lucky enough that the enemies are spread out enough so it doesn't get over-crowded (which my lead to confusion on your part OR it might give Batman an area-of-effect attack where he'll accidentally hit more than one person, which could screw up your flow.)
Here's how I did it. When you first break into the Arkham Mansion there will be a room full of baddies looking for a paper (it's right after the first room where you have to go into the grate on the ceiling). From your position you can glide kick. Do it. It will scare the rest of them and give you time to plan your moves. Now attack each person in a certain order, preferably a circle, so when you get done with the last person the first one you hit will be getting back up and you can keep going. Good thing is that you have a check point right at the start of this area, so you can restart if you mess up. I got to x70 in this area on Hard.