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.