Star Wars: The Force Unleashed

To speed this strategy up even further, do as Lechig stated already, but only kill everyone in the first room you go into (With the Tie Fighters hanging on the roof). Then hit save, change your costume and it will restart you back down the hall from this first room. Each run through this first room should net you atleast 35+ Kills with the Lightning Shield.

The way that worked well for me was:

Force Push open the doors and then charge up your lightning shield before entering, as there are usually 2 Storm Troopers on the walkway in front of you. Run down to the far end on the walkway and grab the Sith Holocron (Unlimited Energy). Jump down to the right and run a few feet from the group of Rebel's standing by the door you have to open after defeating all the enemies.

Recharge your Lightning Shield and this should net you about 8 kills on the Rebels. Then run towards the middle of the room and towards the lift where the Storm Troopers will be coming up from. You should still have Unlimited Energy for a few more seconds and can charge up one more shield before the Troopers come up the lift. After you kill the 8 Troopers who came up the lift, stay on top of the lift and wait for your force to charge up if necessary because the lift will not close or rise up again with you standing on it.

After your force is full, step to anyside (one jump length away or a little more) and after about 4-5 seconds after the floor closes up, another 8 Storm Troopers will be sent up the lift. You'll get used to the pattern and can charge up your Lightning Shield at the right time to jump down and run along the middle of the Troopers, killing all 8 within seconds. There should be normally about 3 sets of 8 Storm Troopers coming up the lift.

And remember, after you kill the storm troopers on the lift, stay on the lift until your force is recharged, if you move away from the lift, more troopers will be sent up and you won't have any force to use on them and risk the chance of deflecting their blasters back at them and killing them before you can kill them with the lightning shield.

This technique should net you atleast 35 kills with the lightning shield on each pass (3 minutes or so).

And just to be sure, SAVE after killing everyone in this first room, change costumes and repeat, this technique will work, It worked for me and very quickly, as you can net up to 50 on each pass.

Additional Note, as you've probably noticed, you don't seem to deflect when you are in the air, so jump everywhere and try not to walk around, as you will waste kills on blaster deflections and not your target of Lightning Shield kills.

This is one case where the DLC is your friend, specifically the Jedi Temple DLC. The first part of the level has you landing on a large platform FULL of basic Stormtroopers, and after dealing with them more Stormtroopers (and one Purge Trooper) appear, ALL of them are vulnerable to a fully charged Lightning Shield. That's how I unlocked this Achievement; I started a new game of the Jedi Temple DLC on the Sith Apprentice difficulty and after letting the Lightning Shield kill everything I saved then jumped off the platform, respawned at the first checkpoint, and kept repeating this over and over until it finally unlocked.