Unable to Unlock 'Invisible War' Achievement

[A.J.]

I've followed every suggestion online but this achievement will not unlock for me. Could I have dome something early on in the game to prevent this or am I running into a bug? I'm saved at Mission 13 in the hall where the turret and the two cloaked dudes are patrolling. Thanks!

[CryoEngine]

what i did was Cloak myself then provoke them to cloak by going Right in their Faces (quick save before doing this) and wait for one of the (or both ) to cloak and then Instantly Takedown.

[A.J.]

Well, I'm a little late reporting my results but I just discovered tonight that I did in fact unlock the achievement the night before posting this message. DXMD, Steam, whatever (?) didn't acknowledge the unlock right away and it sure as Hell wasn't listed in my achievements when I made several attempts in the days after. Yeah, really wish I could get all that time and effort back. User error, software glitch? The world may never know...