A very simple achievement. You just need to get 100 headshots with mondos bullet shot. Upgrading can help but it's not necessary.
You can get this playing though the game normally or grinding it out. If you choose to grind it out, the start of episode 6 Works pretty well. You got 6 guys off the bat, just blow their heads off and restart the checkpoint. The bullet shot neo upgrade speeds up the process.
You can also get this in the challenges. I got a few kills in one of the survival missions.
Hope this helps.