Is it even feasible to complete this challenge in a normal playthrough without an insane amount of grinding? With my current level 30 character, I went out of my way to maximize my kill count - exploring just about every place on the map (and killing whatever I find), entering every cavern I come across and clearing it of enemies, making multiple trips to places where I know the enemies respawn, and most importantly, leaving kill-stealers like Boone out of my party to make sure that I'm getting them all - and I still have only about 500 of 700 kills needed (bringing it up to around 700 total if you add in the 200 for Lord Death).
I really want to complete this challenge, since I think it has the coolest title ever, but man, it's getting to be a pretty exhausting journey.