Below are some jokes, then it gets serious
Poison Their Vegtables
Darth Sion likes Carrots. Darth Nihilus likes Brussel sprouts
Push them off a cliff
Nothing more said.
I don't support cannibalism but this method works.
Enough Jokes! This is how you do it
Use Grenades, the Force and Drain Life.
The force is your friend and so are grenades. Also how did I beat Malak? I drained that muthaf*ckers life! Same thing. Lightning also will help you, and when you fight Nihilus choose your companions carefully.
You want to have "Force Resistance" or "Force Immunity" to prevent his Death Field from causing to much damage. Quite honestly by the time you reach him he will be EASY to kill!
Nihilus on steroids.
FLURRY! USE FLURRY!
The first time you fight him, Kreia will tell you to run away because you can't kill him yet, so you only fight him for a little while. Just keep on attacking then eventually she'll tell you to run away.
The second time you fight him, just keep on attacking and eventually you'll break his spirit and then you can kill him.