To achieve this you can follow the video below to complete the mission as Silent Assassin, Suit Only, Sniper Assassin on Master difficulty. Additionally, you'll also be able to complete a bunch of other challenges by saving and loading as shown in the video for a total of 90,000 XP and unlock...
This will already get you a long way towards the mastery level 20 required, and you'll likely be able to achieve the missing levels by replaying the mission on a lower difficulty and following the different (quite entertaining) mission stories.