Instructions
Train in "Practice Mode" with Darth Vader and memorize every one of his moves. For example, his best line of defense is to press "Back" while his fastest move is to press "Kick" four times. Press both "Punch" and "Guard" at the same time to complete his other move, which you may need to defeat the Apprentice.
Begin the battle by luring the Apprentice into using as many Force Moves as possible. Each Force Move causes the Force Meter to deplete and weakens the Apprentice. This will only allow the Apprentice to use normal attacks, at which he is completely inadequate.
Conserve your Force Moves until the Apprentice only has half a bar of health left. You can hasten this by continuously throwing him by pressing "Punch" and "Guard" at the same time. While a cheap move, it weakens him very quickly.
Finish the Apprentice by using every Force Move you know. Aim to hit him hard and decrease his health as much as possible. You should be able to defeat him in about three to five moves.
Repeat this technique a second time to win the match. Since you need to win two out of three battles, you have some flexibility should you execute any of the moves wrong or fail to execute them.