HITMAN World of Assassination

At the end of each level, when you receive your score and star rating, you will also be given a Playstyle (seen here). The most well known is probably "Silent Assassin" for completing the level killing only the targets and being completely unspotted. There are many of these playstyles to earn, but something to note is that Silent Assassin will always override all other playstyles.

For a list of the known different Playstyles and how to earn them, I found this Reddit thread being put together by /u/wieringa, so all credit to them and those posting in the thread.

The best plan is probably to play through the game as normal, taking note of which playstyles you earn along the way and then if you still need the achievement you will know how many you have left and which you haven't done yet.