Aliens vs. Predator

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.

As "Research Lab" begins, you will see four synthetics gun down some humans. Use your combi stick to kill these four (they will not notice you if you don't move from where you start), save, then reload checkpoint and repeat. I got it easy! Now you don't have to worry about it later!

During the Predator campaign, once you get the Combi Stick kill 20 enemies. This is easily done on humans. Turn invisible and turn on thermal vision. This will give you a very easy view of the enemies. And throwing the Combi Stick doesn't make you go visible again so you can just enter a room and stand on a ledge and throw it at all enemies. Dont go right up to the enemies because then they will see you and start shooting you.