To perform a drop assassination you need to be above an enemy, and either jump or blink directly above them. As you are falling down, you will see a prompt to press , if pressed at the correct time (before you land on the ground/the enemy) you will perform a drop assassination. Now that you know how to do that, to find the empress statue you need to make your way to your office in third mission (it’s a story objective, so you won’t pass it). Once you’re at the office, climb out of the open window so that you’re on a ledge. Continue along the ledge, and you will see the top of the crown of the statue. Blink on top of that and look below to the ground level. Eventually you will see overseers walking around. Once they are just about parallel with you, you want to jump/blink off and drop assassinate one of them. Make sure to make a save when you see one, as this is an easy trophy to mess up. To see it being done, please refer to the video below.