Lucifer Morningstar v. Mephisto in soul-collecting contest
Given a six-month period, and using only cunning and deceit, who would corrupt and collect more souls, human AND superhuman, Mephisto or Lucifer Morningstar?
The only magic these two can use are illusions that could help their plans.
The winner becomes the master of all the demonic realms in existence. The loser gets to boil in a pot of bile next to the winner.