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This achievement can be earned during Mission 3, The Good Doctor.

As soon as you reach the isle that holds Addemire Institute, you will want to make your way to the rooftop of the building. You can get there via the elevator inside the Institute, or by use of Blink/Far Reach. Once you are on the very highest point of the rooftop, make your way to the back of the building and jump on the ledge. If you look down from here and use your spyglass, you should be able to see a guard or two patrolling the balcony below. If they are not in a position where you can reach them, use a crossbow bolt to lure them into a better location.

When the enemy guard is where you want them to be, jump off the rooftop ledge and just as you are about to fall on top of them, hold [ Register or Signin to view external links. ] and you will perform an assassination, thus unlocking the achievement.

NOTE: This achievement cannot be earned during a "No Powers" run; without Blink/Far Reach, you simply cannot reach the highest point of Addemire Institute.