In my honest opinion, Pokemon that are higher than the Champion's strongest Pokemon can not enter the challenge. That way, this can still be pretty difficult.
Is there a level cap for the solo challenge? No, and this is essentially a harder version of the end of the solo challenge because you can't switch to revive. Also, its not supposed to be hard in the sense of 'no pokemon can have an easy time beating the elite four by itself' its supposed to be hard in the sense of 'not all pokemon can beat the elite four by itself easily'. Such as Pidgey, Magikarp, etc.
Edit: Jobrjo, I looked at the front page and my Snorlax was level 75-77 for this challenge. And my name was BLACK.