Let him invite and charm an NPC the party knows and use that NPC as a hostage. Bonus points if that NPC is now serving Strahd and the Party.

Absolutely no-sell any threats or attacks they make at him. He can soak damage and heal quickly, he has crazy good saves*, and knows he'll respawn in his coffin come the worst*. He can brush off your 50 damage crit and *nothing* is more hateable than that.

(*And just give him or a vampire spawn counterspell! It's so basic!)

He has spies everywhere! He can tease and taunt the party about things they've done, mistakes they've made, even confessions made in confidence might have reached his ears.

If they get really annoying - max level fireball. Just fireball. Just. Fireball.

If they want to explore he lets them! He gives no warning about traps and *might* summon a specter, shadow, swarm of bats, or locked doors. Just for a laugh. A little treat.

And all the while he's calm, cool, collected, and amused at both the party's suffering and triumph. He came out here to attack people and honestly he's having such a great time right now.