It is a really easy achievement actually. I was playing my way where I wanted to just play how I thought I would. So the only people I ended up killing were some gang bangers you bust early on in the game for some side mission. I figured that if it was ok to kill anyone, it was drug dealing gang bangers. I kind of wish I knew about the achievement before then, so I could of spared those poor bastards lives for an achievement.

Having played through DX:HR on Hard several times, I've got to say that stealthing through the game is definitely the easiest way to play. The stun gun should be your best friend.

@BoringK: One thing I am wondering about this achievement, is if I've save the game then I kill some people have I lost my chance to get the achievement or can I reload my previous save before I've killed anyone and continue to use non lethal tactics.

It's super easy if you just stealth run your way through the game.