Took me to level 40 to get the badge. I can't believe I had the patience. A couple of hints:
Don't worry about getting any of the weapons other than the shotgun and then the gattling gun. Also you can reload right when the round finishes so you will start with a full clip. Never forget to heal yourself as well.
This badge takes a real long time and gets awfully laggy towards the 100,000 mark b/c of all the blood splattered everywhere. Atleast it wasn't impossible. 1/5