While it's not a requirement, if you choose to follow the video guide below on Master difficulty, you'll also unlock the Silent Assassin, Suit Only, and Sniper Assassin challenges on Master difficulty, as well as, a bunch of other challenges for a total of 90,000 XP that will also get you closer to achieving:
If you do, however, follow the guide on Master difficulty, you'll also unlock:
Otherwise, you can just follow the guide on a lower difficulty and only complete the first assassination method shown (using poison). In that case, don't bother bringing your sniper rifle, but instead take the Electronic Key Hacker to also skip the part that shows how to obtain the Underground Facility Keycard.