Im not certain if this is specifically how this works but I have seen people posting issues about the achievement not unlocking for them.
For me personally - I killed an enemy as his grenade was still physically flying through the air towards me after being thrown, before it landed. This is how it unlocked for me - might be worth trying for those who are stuck.
This was very early on so I can’t say whether or not it needs to be part of a certain kind of job or encounter etc. Good luck