Arch Dukes are the giant monsters that have a shield and sword and act as a mini boss. In order to spawn them , you must raise you wanted level (Red bars in the bottom left corner) to the highest point and just keep killing demons.After you kill a bunch of demons while still wanted, all of a sudden you'll notice the demons will dissapear. Now comes the Arch Duke. Very easy to kill with a fully upgraded Armchair-A-Geddon.However. I would say wait until you are weapon boosting for the challenges because you will kill waaaay over 25 if you want to get every challenge done. You'll save more time in the long run.