I want my elephant back.
Give it to me now!
(Please?)
Me.
lol
Personally what I did was, on my first PoA attempt, just jumped off the cliff for fun, and didn't realize it was a huge jump. Of course, the Elite had postitioned himself right under me, and I got the achievement.
If you can't seem to get the timing right: Get a friend and have him position himself on the edge. Once the friend sees the Elite walking towards the bottom right spot, make a run for it and jump at the end of the cliff. Turn off sprint halfway down, and it should give you enough momentum to land on top of him, or behind him.
If you can't seem to get the assassination: Just try, try again. Melee as soon as your reticle is red. If sprint is still on and you assassinate, it won't work. It is more preferable to land right behind him so you don't bounce off of him, but whatever works, works.