Here's a guide I put together to help you through Nightmare mode (you only have to play on hard for this achievement). The run is 4 hours and 15 minutes or so and I use a bunch of easy to copy safe strategies that should make life in this run easier. You can also use this guide to get the no human kills and no deaths achievement if you like although i'd recommend doing those achievements on an easier difficulty setting. Hopefully the guide helps some of you out.