Chapter 3- The Embassy
Go to the toilets and sit in one of the cubicles. Take control of Aiden and go back the party, look around the room for the Sheik whom was talking to Ryan at the beginning of the chapter (he will be glowing orange). Posses him and head towards the stairs. Go upstairs and head towards the double doors at the end. Walk towards the desk and open the drawer, then push the button in the drawer to activate the portrait. walk towards the safe and activate the palm scanner.
Do not touch any one while Aiden has possessed the Sheik, this will forcibly stop the possession and arouse suspicion, doing so will make you miss this trophy