You can follow the video guide below to complete the mission as Silent Assassin, Suit Only, Sniper Assassin and earn up to 35,000 XP to already unlock several starting and smuggling locations, as well as, unlock the following achievement for you:
With that done you can then follow this video guide below to unlock...
...and complete a bunch of other challenges as well for an additional roughly 35,000 XP.
After that, simply complete the remaining available mission stories and you should have no problem reaching mastery level 20.