This is easier than it seems. You can only get this in the level "Cloud City Trap". You can do this in Free Play or Story. This is easiest to do in Free Play. When you are in Free Play go to the very first place where you get the Stormtrooper Helmet. Once the helmet is on, pause the game and exit to the Cantina, remembering to Save and Exit. If you want to do this in Story mode while playing through here is how. The 'Disarm Troopers' extra can be activated to make this a lot easier. You must get the helmet before you head into the area of the level with all of the lifts that you must use. If you do not get it now, you cannot turn around to get it. Have a second controller come in to activate all of the Panels so that you do not lose the helmet. The part that will be the hardest in this, will be when you have to fight Vader the second time. Have the second controller jump over to where Vader is and then turn from R2 and attack Vader so that you do not have to worry about the objects flying at you with the force. In the final part have the second player attack Vader while the first player stays back to avoid being hit.