D&D 5E Boss Fight Advice

[Quartz]

one of the players is being transformed into a demon and will take on the party.

There was a 3.5E Story Hour where this happened. The PC turned into an uvuudaum IIRC

 

[Benjamin Olson]

PC rules built characters and monster-rules build have very different ratios of survivability vs. damage. A level 20 character will not have the HPs to survive novas from the rest of the party. Also since a PC build doesn't have any legendary action the action economy will be heavily tilted in the party's favor, so the foe will get 1-2 actions total. And without legendary saves itg may not get a chance to take them as it's stun-locked by the monk or action-denial from others.

Basically, this would be a short and unsatisfying fight.

Right, which is why I specifically said to give them legendary actions, saves, etc. in the part of my comment you quoted.