XP in this game comes in the form of money. You need to earn certain amounts of money to level up.
You earn money by killing enemies and completing objectives and missions. You will get more money for killing enemies in more skillful ways. You will get $10, just for a standard kill, but you will get $100 for backstabbing an enemy. Just keep killing enemies and you will likely play through the game more than once for all of the achievements, so you can set this one to the back of your mind as you earn the rest and it will unlock, in time.