Mission 17 (Protecting the Future)
There are lots of opportunities before mission 17 to find cloaked enemies, but they are a lot tougher. So this is your chance to get this Achievement without trial & error frustration.
When you're given the option to stop Marchenko or protect the delegates, choose the right door and go towards the delegates
. Before you do make sure you cloaking upgrades unlocked, especially the "Cloaked Takedown Support
", which you need for this.
You will catch two enemies talking, when they are finished with their conversation, they go cloaked and walk away. You won't be able to see the takedown button commands while they are cloaked so there are two options you have now.
1. Get close to one and smash the takedown button until it registers. (Doesn't always work)
2. Activate your scan vision just before you hit the takedown button. In this mode you can see the symbols and initiate the takedown.