Exotic I've gotten this. the prerequisites require you to hit only an enemy, not ally, and to land the blow AND Knockout, BEFORE you touch ground. also making sure you are on the second platform, not the first one. it appears there is a clear line of where you must be, or sometimes the achievement is finnicky. i find landing with a headbutt to be the easiest, as when airborne, the headbutt tilts you so the head is much lower, allowing you to have more room to fall before touching the ground.
Hope this helps.