This starts very early in Chapter IV (4). You will get to a part where you will learn how to cloak using and have to assassinate a Decepticon by pressing . This isn't too tough to do, as there are very few Decepticons. The achievement unlocks once you make it to the crane lever. Always check your surroundings, do not rush into an area. If you do manage to get caught, simply reload check point and try it again.
Here is a great video guide by Kal_El showing you the path and which Decepticons to take out.