It's an interesting concept. Forcing the player to move quickly. But it doesn't seem to have been made with people that like to get those 100%s in mind. That's always a bummer, as I'm part of this particular category of people.
Having said that, it's still possible to get all kills. It was actually interesting trying to come up with ways to achieve that goal.
Link to Nightmare 100% walkthtough: https://youtu.be/sGdFk1Upw0w