Aliens vs. Predator

On the second level of the Alien Campaign, Colony, there will be a bunch of Marines that will try and kill you. You have to make sure this doesn't happen. Instead, you will have to kill them all.

This achievement can be unlocked on any difficulty, so doing this on Easy will still get unlock it. The marines aren't too difficult if you know how to deal with them. Use your pheromones to your advantage. Watch groups of marines and be sure to see when they separate from each other. You will have to attack them one-by-one to ensure your survival.
Also, be sure to press Y to hiss. This will lure Marines to your direction. This is a great way to separate one person from a group.

This achievement is missable but can be gained through chapter select, however the flag that indicates that it is automatically unlocked through story progress is either wrong or it bugged for me.

I didn't get this on my first playthrough on hard even though I'm pretty sure I killed every marine, not sure you can even progress without doing so.. but I did harvest all the civilians on that run so maybe that bugged it? killing the civs and marines made this achievement pop for me in the garage after chapter selecting for this achievement.