Star Wars: The Force Unleashed

When you are given the choice between fighting Vader or The Emperor, choose The Emperor, defeat him and the achievement is yours.

A video walkthrough for the emperor:

The Emperor is actually really susceptible to Force Lightning. If your timing is good, you can reflect his lightning attacks back at him by pressing the block button when he is charging it up. He'll throw some charged up canisters at you as well, which can be rendered useless by hitting them with Force Repulse. He'll also send out Imperial Senate and Royal guards every so often, which are easier to defeat if you're able to knock them down and impale them.

Whenever he's not doing any of the above, spam Force Lightning until you have only a little Force energy left or until he starts attacking again. Lather, rinse, and repeat until he's dead. I used this strategy on Sith Master and had no problem beating him.

You can do saber attacks in place of Force Lightning if it suits your fancy, but I chose not to since the Emperor will activate his lightning shield without warning. If you're standing right next to him, it'll take away the majority of your health.

So you've decided to go after that evil Emperor huh? Yeah, I can't stand him either. Keep at him close with lightsaber attacks, dashing in when necessary to keep him from running away. When he starts glowing purple and gathers some debris to toss your way, run towards the bottom two panels and stand behind them for cover. Most, if not all the debris, will miss you and hit the cover instead. Once you see that he's out of ammo, rush back and start wailing on him. Keep an eye on his lightning attacks as you draw close, block them and it'll go back to him in some instances. When he summons his cronies, use Force Grip or Dash to send them flying towards the destructive barriers (avoid them, they hurt!) and watch them burn. Follow this pattern as The Emperor goes through his cycle of tricks and you should be good to go.

This is more or less a supplement to everything that's already been said. Rather than talking about beating the Emperor (again), I'll tell you how to get both the Redemption and Revenge endings from a single save by fighting both endings within a few minutes of each other. "But wait," you say, "I have to do two playthroughs anyway to get the Sith Master" ending. Sure, that's one way, but if you feel like two achievements now, and getting that other way later, read on.

Play through the game to the final save point. I chose the first Darth Vader fight, but you can likely take it one further to after beating Vader the first time, and save it there as you have to make your choice. At that final point, save and exit out of your game.

Go to the dashboard and copy your game save to another medium (such as cloud, hard drive, memory stick, just wherever you haven't been playing it from). Return to your original game, and finish off the Emperor for Redemption.

Once you've viewed all the cinematic goodness that one has to offer, exit out of the game, load the save from the other medium, and finish off Darth Vader for Revenge without even replaying the entire level to get the other ending.