This achievement really isn't as hard as some make it out to be, I got it first try counting the enemies I kill on the way through each part of the mission.
1-1 There are two enemies that you have to walk past because you cannot kill them, these two don't count.
1-2 There are three enemies in this level, you'll take out the first as soon as the assassin lying on the ground gives you his sword, the 1st comes running towards you. the 2nd is at the end of the level before the big gate ending the second part of the mission, once you kill the 3nd enemy Don't Advance, turn around and backtrack a little and you should see the enemy running for you. Kill the 3rd and last enemy.
1-3 There are 12 enemies in this last part of the mission and none of them are easily missed. thoroughly make your way through the last part counting the enemies as you kill them as I did to make sure you aren't missing any. There's not much else to say for this last part, it's quite simple to fine the enemies on this part, they all run towards you anyway.
Most miss the 3rd enemy on the Second part and are confused as to how they haven't unlocked the achievement.
I hope this helps! Thanks you for reading.