Did you kill the scientists, as SvD.HaZe said? What about the creatures in the veil? There are no special enemies on higher difficulties, because I got the achievement after defeating the final boss on normal. As far as I can remember, there aren't a lot of enemy types you can pass by in this game either.
My advice is to roam around mid-town and downtown (if you can do so without freezing), killing all enemies that you see. Then replaying the final boss fight as he is needed for the achievement.