For players that want to earn this quick. (Can also be done through natural progression)
This achievement can be earned as early into the game on the 'Moscow' mission.
Using the same trick as Metro Redux where you could get 5 takedown stun/kills then reload last checkpoint and repeat another 5 stun/kills and the achievement tracker will continue to go up even though you killed the same people.
This won't contain spoilers but you'll get to a stealth section outside in the snow with a couple patrol groups.
You'll know the one as your companion will say …"Don't turn your flashlight on either, they will notice you"
Once you're here you can quickly knockout or kill these enemies then reload last checkpoint and repeat.
It doesn't matter either if you get caught in the process as the button prompt
will still come up if you rush them.
This is a nice area with a cluster of them making it easier to do in rapid succession.
I made a Stealth clip for example
I went behind the barrels and knocked out the guy by the light and shut them off to make the stealth approach easier.
Sorry, this Game Clip has been removed
And another where I got caught but was still able to take them down, I died in the process but it still counts the kills/stuns.
If you're feeling brave you can also get this achievement here too: