Simply waiting around in the cantina isn't going to be very effective for this. I've found an easier way to do this. Go to _any_ level and start it in free play mode. Choose Darth Vader as the character you want. Once in the level, quit back to the cantina. You will bring Vader and another character back to the cantina with you. If you are controlling Vader, switch to the other character with you (get near them and press 'Y') and go back into the level. Start up Free Play mode again, and at the character select screen the green circle (denoting the secondary character to be used in the level) will be around Vader. Now you can choose whoever you want and be guaranteed to get Vader in the level with you. Personally I used the Episode III Anakin, but it should work with the Padawan version of him too. Do not choose the Episode I, little Anakin, it won't unlock the achievement. Once the level loads hack Anakin to pieces with Vader, and pop!