The skill is unlocked via
Aragami Skills tree menu (1st one)
its in the 1st line of skills and the 2nd option
requiring 3 skill points (
scrolls)
the first one
Shadow Leap cost me
1 point (required to progress & this achievement)
the 2nd one
Ledge Kill cost me
2 points (required for this)
Then simply Perform a Shadow Leap (
) next to a guard on a ledge above you (i.e.; a climb) that will result in an instant kill & achievement.
Youtube video courtesy of ted x RAD0 x
(video solution is @ 40 seconds in.)