Star Wars: The Force Unleashed

This achievement will come quite naturally during your playthrough. However, if you are impatient, you can grind this out on the Prologue level with Darth Vader.

After you destroy the first large gate in the level move halfway between it and the group of trees beyond it. Kill all the Stormtroopers around you using whatever method you prefer. More will continually spawn next to you in groups of 5 which should give you more than enough to make this achievement. Note that if your camera is facing the gate they may not spawn. Simply spin the camera around to face towards the forest and the Stormtroopers will spawn immediately.

Personally though I would recommend just playing through the game naturally as there's no need to waste the time grinding this out. Enjoy the game, there are more than enough Stormtroopers for you to get this achievement without even trying.