After completing my 2 playthroughs of the game and doing the challenges, I'm still grinding this achievement. I'm using Chapter 1, Level 5 Train Wreck. I have a maxed out group but I am using Yang because he's simply the best for this.
I'm getting 80-100 kills each time. Play on casual, and try to not worry about the helicopter at the end. Waves of enemies come periodically and I would slice and dice through them and throw knives at the snipers, then cycle through other characters to keep them from firing on the helicopter, and wait on the next wave. I'm working on it as I write this and hope to have it soon.