Has anyone heard of this achievement being glitched or have any tips? I know I am well over 20 but I decided to keep track all over again to make sure. I’ve been playing with a friend of mine and in three matches on Shatter I have gotten a confirmed 39 detached turret kills while playing in the Heavies playlist. 
Many of the DLC achievements have been glitched since the release of the Champions bundle, but I never heard of this one being glitched.
Yes, it is glitched. It is the only one I need. I have about 100 kills with a detached turret on Shatter and it will not unlock. Please post in the Halo Support Forum. 343i have a thread tracking DLC achievement issues.