First, you need to have beaten the game, this works on any difficulty. Depending on which side you chose will determine which boss you will be fighting over and over again. Now for what you have to do. Enter Plane of Tranquility (evil) or Plane of Fear (good), fight the boss determined by your side. Once the boss is defeated, you can loot all the armor or gear dropped off by him. After they have died, click the "start" button, then click "End this mission." When the Planar viewer is brought up, select Plane of Nightmares. Wait for Plane of Nightmares to load up, once it has loaded, click "start" again and select "End this mission" again. When Planar Viewer pops up again, select Plane of Tranquility or Plane of Fear. When the area loads, you will see it has refreshed, and the boss will be there again, you will be able to kill them and repeat.