This achievement can be unlocked in the Predator's campaign or in multiplayer. In multiplayer, I believe the kills must be in Ranked matches. If someone can confirm this, pleas let me know.
I did this in the campaign. After acquiring the Combi stick (after defeating the Praetorian), I just started abusing the Combi stick on Synthetics. I was also playing on Easy mode so difficulty doesn't matter.
You can use the Combi stick to kill any enemy but humans and synthetics are easier targets than the Aliens. Also, aim a little lower than you wish to hit as the Combi stick always hits a little higher than where you aim.