Star Wars: The Force Unleashed

I did this playing on Sith Master..

Easiest way I found to beat him (although maybe not the quickest).. is to block, letting Darth Vader Slash away.. Cowardly I know, but gets the job done.. while he's getting nowhere, hover over the B button so when he grabs you you can bash it quickly, then as you escape, get away & ready your block again as soon as you land, as he can land a cheap one..Also, (and this is our turn to attack) When he readys a red force ball, quickly jump over him, being sure you are actually behind him, as Vader seems to instantly hit you even if your to his sides, or above him, it really has a wide hit range, annoyingly.. once your behind him, as soon as the red sphere isn't behind him, use the XXX combo & then block when you see the 3 strike hit, or he'll hit you.. and keep Vader's back away from the wall, so that you can jump behind him when needed..

So in short
Be patient, Block, always block, until either
A) Vader grabs you, bash B, get away and quickly guard
or B) Vader charges a red sphere, get behind him quick & XXX and then guard

For Revenge and redemption
taken from x360a

You receive these two achievements for completing the game as the Dark Side or the Light Side on any difficulty. The light side ending requires you to defeat the Emperor. The dark side ending requires you to beat the battle damaged Darth Vader. Once again click here for strategies on how to defeat either boss. See below for more detail on how to obtain these.

After the cut scene where you are asked to help defeat the Emperor you are given a option to fight and defeat the emperor for the Light side ending, or go and slay the recently battle injured Darth Vader for the Dark side ending. To fight the battle injured Vader, jump off to the right of the platform that would lead to the Emperor shooting his lightning. Vader is on the right of the screen you will see a pretty small red glow from his lightsaber to show where he is, defeat him and enjoy the Dark side ending.

NOTE: If you don't want to do two playthroughs of the entire game, you can replay the last level, Death Star, and simply do the ending opposite of the one you picked before.

Another great way to take him down, and quickly, was to do a quick charge into him, attack three times with the x x x and finish by poping him up into the air. He'll more than likely block the first three attacks but if you're quick with the poping him into the air, you can then juggle him for more damage and then follow up once he hits the ground. He can't actually block the shot from the ground putting him into the air. I was able to make quick work of him that way.

I found that force lightning is effective against him. As mazrael said block a lot. Also, if he starts to charge his force push start hitting him with your force push. Fire rapid shots at him. You should enter a moment where your force powers are equal. just keep hitting B (i think it's B) and he will be blown back. Then hit him with force lightning as you close the distance to use your lightsaber. When he has you in force grip it is hard to get out in time. If that is the only damage he really does to you then you should be OK.

This is more or less a supplement to everything that's already been said. Rather than talking about beating Darth Vader (again), I'll tell you how to get both the Revenge and Redemption 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 Darth Vader for Revenge.

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 the Emperor without even replaying the entire level to get the other ending and Redemption.