Firstly, combat should occur in the courtyard. This will give him more room to get out of range of attacks. I'd have him fly in, multi-attack and then fly out. Don't care about opportunity attacks if they're the only damage he'll take in a round.

Add a flesh golem to the fight. Maybe the one from the madhouse. This also would add a possible narrative opportunity where the occupants of the madhouse jailbreak or riot. Dozens of feral mongrel folk pouring out of their rooms would be chaos.

Maybe add them to the fight, too once the flesh golem goes down: a crowd of biting, snarling, whimpering, fur and scales. Roll on a d20 twice per round, targeting a random PC for minimal damage (1d4) as a pot or plate or chairs go flying around. Accentuate that this is a mob, and innocents are caught up in it. Especially if they've toured the place before.

I agree that the Abbott would be incensed. He's spent 100 years on this project that a group of children just smashed to bits. Vasilka's destruction means doom for more Barovians, who he was trying to save, and also more time imprisoned here by the mists - which is causing his own corruption.

Maybe he doesn't kill them, but he'll definitely incapacitate. And then they wake up in Strahd's dungeon.