There are 13 achievements in Star Wars: The Force Unleashed that you can get using Vader on Apprentice in the first prologue mission via the killing of Stormtroopers alone. So if you don't want to stop after killing 500 troopers, go for it. Stormtroopers are dumb, and weak, they barely ever attack Vader, unlike Wookies, making them a safer target. The 'How To' for this and the other achievements is below the list. I basically focused on a Force Power achievement at a time, and in earning those achievements also earned the 3 stormtrooper/imperial killing achievements.
3 achievements for killing Imperial forces:
and 10 achievements for using Force Powers:
(Face Target at short range, press 'Y', you will force grab the target, choke him and toss him sideways)
(At short range Jump by pressing 'A', While in the air press and hold 'X', you will slam the ground and nearby enemies will bounce up in the air taking damage)
(Face Target, tap 'B', you will blast in the direction you are facing, sending targets flying)
(Hold Left Trigger, tap 'B' near Targets, You will generate a 360degree force blast scattering/killing enemies))
(Face Target, Hold Left Trigger, tap 'X', You will throw your lightsaber at the target, killing it and nearby targets)
(Face Target, Hold Right Trigger, push and hold either stick to the left or the right and release right trigger, You will lift the target with the force and throw it violently in some direction)
-----------------------And Here Is The 'How To'---------------------
Start a new game, select the difficulty of 'Apprentice' and make your way through all the cut scenes to the start of the prologue level on the wookie home planet.
The first part of the level has you walking past a few stormtroopers and wookies. Blast 'em if you want, test out Vader moves, get comfortable.
Very quickly you will come to a big wooden barricade and the onscreen instructions will ask you to press and hold 'B' to smash it.
Do so. Then walk through the hole. Shortly after that barrier is a path. You should first see, almost directly after the barrier you smashed, a big(huge) tree on your left. Just past that tree on the left is a boulder that you can pick up and move with the force.
In this area are a few storm troopers, and at the far end are at least 3 wookies that will try to jump down and attack you. Kill everybody in the area, wookies and stormtroopers. Head forward and kill anyone standing in the narrow path between the trees and boulders; where the wookies had jumped from. Then turn around and head back. On the side of the Path, near the liftable boulder and huge tree, four(4) stormtroopers spawn, not always all at once, sometimes you kill them so fast it's just one at a time spawning.
You can kill these stormtroopers before they start to spread out using one of the above listed methods.
After you kill them, as long as you are further up the path than their spawn point and the camera is mostly facing upslope/toward the point where the path narrows they will respawn. FOREVER, 1000s of times! And the nice thing about the Stormtroopers is that they will barely ever shoot at you, even though you are killing them.
For some moves I was pretty much on their spawn point (ie Grappled, 'Y' button choke), and for others I was upslope facing their spawn point with the camera looking away from it at Vader's face. I just kept blasting and they kept respawning, even though I couldn't actually see them (ie Pushed, 'B' button force blast).
These four troopers will keep respawning and you can kill them forever, you may have to readjust your position or rotate the camera around or blast any troopers that have got out of the spawn point once in a while in order to keep them spawning efficiently. However it's easy; mostly these are aim, kill, and repeat type achievements. This is not always the fastest spot to get some of the 13 achievements, just the easiest to find, and safest/simplest to use.