In the "Freeform Training" mission, get the rat poison on Deck 00. Put the poison in the wine glass by the bar on Deck 01 and let Norfolk drink it (the guy in white suit). Norfolk will then get sick and go to the toilet. Follow him and assassinate him, then put on his disguise. Next, go to Deck 02 and talk to the target. Follow him to his office, then assassinate him from behind once you are alone to get the "Unexpected Guest" achievement.