Instructions
Turn on "Perfect Dark" and select "Combat Simulator" from the main menu. Select "Challenges" when prompted.
Select "Weapons" from the game setup menu. This will show you a list of the default weapons.
Press "A" and a list of weapons will appear on your screen. Scroll through the list, selecting the custom weapons you would like to battle with by pressing "A" again.
Press "Done" to start the battle. Your character will start out in a room with either a gun or ammo. Pick up these items by walking on them. Only the custom weapons selected in step 3 will be playable.