Rule reference

[McHam92]

If {Prey Upon} is used against a creature with  {Spirit Mantle} what happens? Just to be sure. It's an illegal target, right?

[Keyeto]

It's not an illegal target, but protection from creatures will prevent the damage from being done to the creature with {Spirit Mantle} on it. It will essentially end up as a one way fight, since only one creature (the one with the mantle) can deal damage to the other.

[Wally]

However, if the fight was caused by  {Nightfall Predator} or  {Ulvenwald Tracker} then it would indeed be an illegal target (due to the protection from creatures) meaning it could not happen.