This is very simple to get, but quite tedious.
You can only obtain this on the prologue level, where you play as Vader, so must start a new game if you don't already have it. It is also not possible to replay the prologue without starting a new game so make sure you get the achievement before completing the prologue.
There is a section in this level with a couple of rope bridges (you see a TIE Fighter crash through one of the bridges so you are forced to walk over to the next bridge along).
If you go over this first bridge onto the first round platform (that is covered) you will find that the game spawns an infinite number of Wookiees from the opposite platform.
As long as you do not move onto their spawning platform you can kill as many as you want.
This also means that you can kill them using all of Vaders force powers to work towards the majority of the "Mastery" achievements (cannot do Lightning or Lightning Shield as Vader does not have these abilities).
It may take a while but it's the easiest way of obtaining these achievements.