To get this achievement, you need to complete all the main missions without killing or knocking out anyone after the one mandatory knockout in the prologue. If you are like me and aren't great at this game, here are some tips.
-Play on easy. The enemies are far less observant.
-It is easiest to do this as a mostly stealth playthrough, but if you get detected you can run away pretty easily in a lot of areas. Your priority should be to complete the main objectives. Getting treasure shouldn't be a priority unless you are trying to do multiple achievements on one playthough.
-You can use choke arrows and flash bangs if needed to get past an enemy.
-Use a lot of water arrows, and be sure to keep a lot of them on hand. Also, be sure to keep a lot of food.
-Buy the wire cutter and wrench as early as possible. Then focus on upgrades like lockpicking, damage resistance, the one that makes food have more effect, etc.