This is a simple achievement but does not explain what you need to do.
In order to unlock this trophy you simply need to kill a Mudokon using a environmental trap. This can be done several ways and, also within the first few minutes of playing the game.
In the first level, you will see a lever and a Mudokon who happens to be cleaning a wall. Just pull the lever and the Mudokon will fall to its death.
The trophy should unlock soon after.