Glitched beyond words.
Syndicate Gangsters don't count towards this achievement, after much experimentation the only kills I found which count are:
Level: Final mothership
Stalkers where they guard/accompany Eternal Worshippers or Warmonger Employees.
One consolation is you'll get credit towards the eternal worshipper or employee kills too.
Given the rarity and glitch best to use the console save / load exploit the level you find the combo of Stalker / Eternal Worshippers / Warmongers.