If you mean would the Demogorgon stand a chance ... honestly its not looking good for him.
Here is the issue, Demogorgon's Gaze is laughably bad for this high a level. All a player has to do is suffer "blindness" against demogorgon in return for complete immunity to the save.
In other words, 1 player in 5 suffers a minor inconvenience (his party tells him what square Demo is in, and he attacks at disadvantage) vs Demo spending his entire action.... its no contest, any sane party will do this in a heartbeat.
So then it comes down mainly to a brawl. Demo has 400 hitpoints, a 22 AC, and some decent elemental resistance. While the tentacle "I can't heal the damage" looks scary, 28 average hitpoints is garbage damage at this level. He has no special movement, no fly, no teleport.
So if this is 1 monster vs 5 party members...the party members slam dunk this. They just wail on the poor demo until he is dead. Maybe the demo does some decent damage to one of them....but he is going to paste in a few rounds.
Honestly the Demo is just a poorly designed monster for his CR...he has none of the needed tricks to hurt a party of such high level, and none of the special defenses really needed to stand against them. If you want to make a subtle change that would ramp him up a bit, give him a 3rd legendary action, and remove the "avert your gaze" silliness of the effect. Then he at least has some teeth....even then my money would be on the party, but at least it would be a fight.