this trophy can be rather annoying to get, mainly because there are many situations where it feels like you can't progress past certain human enemies because the level design is badly programmed.
i got this trophy on my first try even though i thought i had failed it. i knocked out a guard near one of the train tracks and i hit him with the trolly you control later in the game and his body twitched and moved, which led me to believe i just killed him.
another instance is where i knocked out a few guards in a room, and the other guards in the adjacent room, spotted me and threw a couple of grenades and killed the guards i had knocked out earlier.
regardless of my fear of not unlocking the trophy at the end of the game, i carried on and i unlocked it to my surprise. so as far as this trophy is concerned, any in-game mechanics that kills guards you've knocked out doesn't void the trophy in any way. as long as you don't kill them with your own weapon.
the last level where you fend off a bunch of soldiers with shields and a flamethrower guy behind them, those do not void the trophy either, so you are free to kill them, preferably with the minigun standing behind the last barricade.
so even though you might think you screwed up this trophy, just carry on anyway, because theres a high chance you didn't mess it up. still, be careful and be cautious going for this trophy, because if you accidently kill any human enemy with your weapon, throwing knifes or knife execution from behind, you have to restart the entire game, choosing new game at the main menu. you can't reload checkpoint, it won't help.
if you need more information, send me a pm on psn or on here.