On Episode V, go to the Cloud City level. Defeat Darth Vader for the first time. After that, you will be playing as Leia, Lando, Chewy, and R2 again. Find and rebuild C-3PO and make sure your partner is C-3PO. Run around, fight off some troopers, and if they do not do enough damage, then you can. As soon as you see C-3PO without arms and one leg, shoot him one more time. This will not work if the troopers do it. He will come back, but all you will see is his shadow on the ground. After a few seconds, he will be visible again. You can repeat this as many times as desired.