The key for this acheivement is to avoid being detected for such a period as the marker goes red and the enemy then start shouting/shooting.
Obviously difficulty has a bearing on how quickly you are detected, so if you are just going for this acheivement, the lower the difficulty, the longer you get as "grace" before detection.
Equally you do not need to play "ghost" style, albiet it makes most sense. That said, I used a mix of hand to hand take downs on single enemies to build my execute meter, and then used this, with the silenced pistol to clear areas with multiple enemies.
If you do get detected, then you can reload, as the key here is the autosaving at checkpoints - as long as you were not detected in the current go (excluding reloads) you will be ok.
Lastly, if you are still struggling, a good option is to progress the game and use funds to unlock gadgets/upgrades to make the job easier - good examples here are the crossbow with shocker/gas bolts, the gas grenade, and clothing for enhanced stealth/noice reduction.