Since you're keeping things at groud level where the Monk can go straight to melee I'd give it somewhat better Con saves, because stunning strike will use up the legendary resistances too fast, and a stun-locked creature is not a satisfying solo enemy for a party. If, on the other hand, it was flying and it was a multi-round struggle to get the thing in melee with the Monk then I wouldn't worry about it.
What minions does the BBEG have? How can the PCs prevent him escaping via teleport? Or simply flying away.
BTW if this guy is a fallen celestial, why is he still resistant to Radiant damage? This is a significant nerf on the Paladin.
So, we'll be having my homebrew campaign 'Grand Finale' in a few weeks with an all or nothing fight (no fleeing, no surrender) against the BBEG. It's going to happen at a big city square and the BBEG will keep the fight at ground level, for campaign reasons. My question is: how balanced is it for a end-of-campaign epic fight against a 9th level fully rested party? Any help is appreciated.
The party (all 9th level):
Elf Wildfire Druid
Halfling Celestial Warlock
Dwarf Battlemaster Fighter (shieldmaster feat)
Human Drunken Master Monk
Human Homebrew Paladin
Edit: forgot about the paladin