This achievement will take quite some time unless you are specifically going for it, so be prepared to put some work in! To perform a takedown, you have two options. To SNEAK or to STUN.
To perform a takedown while sneaking, all that you need to do is get behind an unsuspecting enemy, wait for the takedown prompt to be available to you and then attack using
. To perform a takedown on a stunned enemy, the most common way you will be stunning an enemy is using critical shots. To perform critical shots, you will need to perform ranged attacks to the head of an enemy until they become stunned. Once stunned, it's just like the sneaking takedown except you won't need to be behind them! Just head to the stunned enemy and follow the takedown prompt to successfully takedown the enemy. You can also boost your "stun ability" by purchasing skills, but I did not find this necessary. It would be completely up to your own preference!
My suggestion would be to just play through the game as you normally would, but taking the time to watch out for takedown opportunities. Any enemies on a roof? Try and sneak up on them! Enemies all around an area with their backs to you? A perfect chance to get more takedowns! Using your ranged attacks and you get an enemy stunned? Another great and easy chance to get another takedown! Just do your best to play through the game watching our for any opportunities to takedown enemies.
If you are looking for a location to grind, the Naval Base has plenty of different enemies you can perform takedowns on that are also conveniently required for some Karma tasks. That's definitely the best spot to grind at.
Along the way, you will also unlock the following achievement:
If you would like a video guide, I've made a quick video that essentially illustrates what I have written above.
Hope this guide helped! Please let me know if I missed anything or got anything wrong! Constructive criticism is always welcome.