Once you earn 20 gold bricks (by completing levels, earning True Jedi status on levels, finding all mini-kits in a level, purchasing from cantina), you can build the entrance to bonus level 4 - A New Hope. Upon completing this bonus level in Story Mode, you will unlock Darth Vader as a playable character.
Now that you are controlling Vader, wander around the cantina until you find Obi-Wan (any incarnation) and switch to him with the Y button when you get close. Attack and destroy Darth Vader for the Achievement.