Regicide Trophy in Aliens vs. Predator
Defeat the Matriarch
How to unlock Regicide
This is story related and cannot be missed.
At the end of level 2 there will be a boss that you have to defeat. Simply get through this area and defeat the boss to earn this trophy.
You need to watch out for the Facehuggers here. I recommend you fire at all enemies whilst running backwards, this way you will always be quicker when defending. Always try to use the grenade launchers because they deal the most damage, and always run over the stims to give you a constant supply of health, then just follow your instructions on how to beat the boss and you should be fine.