A chance of beating, or a chance of surviving?
If they got to defeat it, then what you're talking about sounds a little tough, though I'm not enough of a number cruncher to know for sure. However, if the PC's have certain information that will help them slay the monster (which if it wants to be transformed into the shadow/thingie, it may well provide itself), it may even the odds a bit.
But in order to simply survive the encounter, the characters may run into a significantly stronger demon, in which case they'll have to run, negotiate with it, call upon gods, or do some other non-combative action to live another day.