Other standing guides seem to work as many people upvoted them, however I tried everything exactly as described on those for about 5-6 looong hours with 0 luck.
Then I started reading some comments on youtube videos of people unlocking this and sharing their experience.. and the single tip that worked for me was to do it on LEGENDARY
I tried Sierra 117 at checkpoint Alpha, at checkpoint Charlie for 5-6 hours on normal with IWHBYD skull on and could not get it done.
Then I again loaded up Sierra 117 this time on legendary, at Alpha, and I was lucky enough to get it in roughly 10-15 minutes!
On Legendary your chances to get the dialoge increase because you get to face 7 grunts in the first fight (4 behind fallen log and 3 coming from the back and to the left), opposite to normal difficulty, in which you get only 4 grunts (behind fallen log in the river). This way your odds increase by 75% for killing 3 more grunts per reload.
And it was actually one of these extra grunts from the back that got me the achievement.
Sharing my experience and the video where it has popped for me. You will not see the achievement popping on screen due to Xbox DVR recording limitations.http://xboxclips.com/el+tuaya+arg/2d0f6e8f-2ec1-4113-ab9a-39...
3:24 into the video is where it happened. Summing up:
Mission: Sierra 117
Checkpoint: Alpha (first fight)
Skulls turned on: IWHBYD.
Hope this helps. This is not a "new solution" in any way, just that the time it took me on legendary is absolutely ridiculously small compared to the time spent doing it on normal.