Alternative location for Flipyap achievement: Load up Sierra 117 check point Charlie on Normal difficulty with only the IWHBYD skull on single player.
Make your way through the level (less than 2 Minutes) to the cut scene ‘Quid Pro Quo’ once cut scene has finished just follow as I do in the video and lob a grenade at the Jackel on top of the building below jump down into the building and only kill the first and second gold grunts I’m pretty sure the second one is Flipyap once he is dead another grunt will go on to say how you killed his brother and the achievement should pop if not just restart last check point and make sure you follow the video, don’t worry if the Jackel is not in the same place as in video the purpose of the grenade is to get the grunts to run toward the door of the building.
This is my first guide so please let me know if I can improve on it in any way.
EDIT: OK so I just tried this on a second account and it took about 10 reloads before I got it. I was trying different methods to that of what I originally did in the video with success. It appears that once you have killed Fliyap that his brother needs to see his dead body with you near it so I would suggest just going in the room down from the cliff top killing the two Grunts within said room then trying to entice the other Grunts in by hiding in the far right corner when looking down into the room from the cliff top then popping them off slowly one at a time once you're sure they have seen the two..three..four etc dead grunts in the room as it is possible that one of the grunts entering the room is in fact Flipyap and his brother sees you put a cap in his head. Anyway I hope this additional info will help those who are still having trouble with this achievement I have upload the video for this run (second video) Please let me know if this is a better solution than that which I already posted and I will edit accordingly.