This objective becomes after you encounter La Touche.
When you make it into the room where you can encounter Le Roi des Thunes, head over to the right. Slide through the hole in the wall and go over to the right. You will find a pair of guard there. Just load up the Phantom Blade and hit them both with it.
Once you make it on through to the assassination of Le Roi des Thunes and are on the escape, this is the ideal time to complete the challenge. If all options have been taken, you will be shrouded by smoke. This will ensure that the guards below will have little idea of where you are.
- 3 Air Assassinations
- 2 Phantom Blade Kills