I recorded myself assassinating the Elite at “emile cliff” three times. On Norm, Heroic, and legendary. I heard you have to turn off sprint before you do the assassination, so I did that and still no unluck. I also heard it had to be the “land on the head” animation. I did that as well. What Else am I missing? All three vids on my file share btw. GT TimZaRob
There is no right or wrong way, it’s just glitched. Some people swear by the “turn sprint off” method, but I got mine when Sprint was on.
I don’t even want to think about how many successful attempts it took me before it registered. Just keep at it, is all I can say.
I am doubting the SPRINT dogma anyway. This guy had it on longer than I did. http://www.youtube.com/watch?v=OSXwr-JiqhU&feature=related and he still got it. During the video it said it took him over 4 hours. 
When I was trying to get it and it wasn’t working, I looked up basically all the info I could, tried it, nothing helped. Landing on his head a split-second before the assassination, not landing on him at all, having the pistol equipped, having the SMG equipped, etc. etc.
It’s just really buggy. Also, like I said with the sprint thing. My assassination animation wasn’t the head stomp one (Which some swear by), I think it was the neckbreak.
Just look at the bright side. . .It helps onyx a few of those campaign commendations. 
Also adds a crap load of deaths.
It took me hoursssssss to get this one and I just couldn’t land on the darn Elite. He would always step off to the side at the very last second or something, RAWR! Then the time I finally landed on the Elite, my battle bud landed on my head knocking me off. >.<
I eventually got it though. Just by doing it over and over and over. 
Well, I’ve unlocked almost all other achievements. I guess I’ll keep trying.
OH HAPPY DAAYYY! I assasinated him 3x in a row and the third time registered. (and yes, I sprinted the whole damn time)And it says earned 12/7/11, sweetness.
The “Emile Cliff” os ONLY one way to get the achievement.
For those still looking and strugling with it, here is a easy way to get it.
Nighfall, Legendary, after you assasinate the first Elite, there is another one below you. Read the Achievement…and you will see why I have gotten it here…6 times on the first try for me and 5 friends.
Wait for the Elite below to line up under you, and then pounce on him…Achievement unlocked and every body is happy:)