If you are stuck in a fight you can’t win and know that the demon you are fighting will not leave of its own volition, then your spellcaster might be tempted to end the fight early and cast banish. Since the mortal world is not the demon’s own plane of existence, you can attempt to banish them. The spell will succeed without any checks or saving throws by the demon and they will not return so long as you can keep the spell up for one minute (10 combat rounds). Make sure to avoid getting hit or anything that could damage your concentration, and then the demon will not return. Well, they might come back, and chances are they will target the spellcaster first to make sure that same trick can’t be pulled again!