Instructions
Start the game's offline "Story" mode.
Play through the game's story.
Go to the Planar Gateway after you beat one of the game's "Plane" levels such as the Plane of Nightmares or the Plane of Torment. When you go to the Planar Gateway after beating the plane a first time, you will see a new area that you can enter to the right or left of the plane. This new area is the medal round for that plane.