To get this achievement, you have to Poison an NPC, or an enemy with your blade, then perform an Air Assassination on them.
Easiest way to do it, is find a guard where there is something you can climb on above him.
Poison him, climb up, switch to your hidden blade, and perform an Air Assassination.
Make sure to not do a regular Assassination, as that won't work, and it's difficult to assassinate someone who's poisoned.