Mission R-05: Very beginning. Provides an excellent farming location if you do not have this achievement at the end of the campaign (like I did not).
After the initial battle against a few cyborgs, there is a potential un-ranked conflict with 3 patrolling Gekko (bipedal walking creatures). I found that with a fully upgraded polearm, and wearing the Wig B, I was able to initiate the prompt for executions (Y+B) by hitting the Y attack only once with Ripper mode engaged. Beware not to hit them too many times, or the prompt will go away.
After killing all three, you may restart from the pause menu without having to re-kill the initial wave of cyborgs.