To get this achievement, you must kill 100 enemies inside the Hivebuster Venom. Now you may initially believe "Hivebuster Venom" means where you're taking damage, but it actually doesn't. This is why the achievement is called "On the Razor's Edge" because it's referring to being in the light green area that comes before the dark green area that gives you damage. Just refer to your lower right mini map to see when you are standing in the light green area. You also be able to see venom particles floating around the room while in these areas, but as I said before, you will not be taking damage yet.
My achievement tracker oddly didn't work for this achievement, so that's why I thought I had to be doing something wrong, but just keep getting kills in the light green areas and you should be fine.
Here is a picture to better understand what "light green areas" mean: