There's a good chance that you'll just unlock this naturally, so you can easily leave this one for late game.
This can be done already on the very first level, it will just take a couple of minutes. Get all the loot, (killing guards is optional but makes getting the loot easier) then get spotted and smacked by a guard or fall to your death.
If you want to get this later in the game, the "Level Scanner App" goggles upgrade will tell you when you've got 100% of the loot.