Well ain’t that luck for you. Over the weekend I was trying to get this achievement for the longest time, in total about 3 hours until I finally gave up. Today, I load of the mission “The Pillar of Autumn” 1 time in order to complete it solo on legendary and I get it on my vary first try. Incredible. Don’t ask how it finally unlocked because I don’t even know. I was doing it the same exact way I did it the other 70213834 times.
Update: I see a lot of people visiting this post probably in hopes of figuring out how to get this achievement to unlock. Honestly all I can say is that I attempted this too many times to count and believe that this is a luck based achievement. Anyone who says that it is skill based is lying to you. What I did when I finally received this achievement was no different than my previous times.
Step 1: Load up “The Pillar of Autumn.”
Step 2: Jump off while holding LB to get some distance off the first cliff.
Step 3: Land just a bit behind the Elite and hold RB to execute an assassination.
Step 4: Pray you hear the achievement pop up. If not, reload checkpoint and repeat over and over again.