This is done in the Marrakesh summer bonus mission. You will need to kill Mendola with a crystal ball, by acquiring the fortune teller's disguise. You will need to electrocute Kong, by acquiring a security and waiter disguise, as well as a key to the backdoor area of the cafe. Finally, you will need to poison both of them on a pipe. Lethal poison can be taken in your loadout and is unlocked by reaching level 10 mastery on Sapienza.
Here is a video guide with English commentary for this achievement!
Here is a playlist of guides for this game!
https://www.youtube.com/playlist?list=PL-HeW6XkKdUK9Qh-6TvL3...