I put my notes on running Strahd (as is) in a Strahd thread, but in summary, in his castle he's near invincible because of his Lair ability to run through walls.
He's a genius tactician and general. He can run through a wall, regenerate 100%, and come back. I played him hit and run. He'd come running through a ceiling, floor or wall, taking an Attack of Opportunity (maybe) to strafe a PC, preferably a caster. If he misses, he just comes back after he's healed up.
If the PCs tried resting, he'd summon minions to create enough noise and havoc that they couldn't (Leomund's Hut helps, but they'll just be waiting outside). Same if they tried to wall themselves up somewhere strategic. He can wait them out. He's eternal.
That all said, if you play him right, even with artifacts in hand, he should be unbeatable. So, like I believe bosses should be, I made him a puzzle to be solved. There's all this backstory and history about Ireena, a diary, even a novel written about it all that didn't make the module cut. Whoopity do, perhaps? But what if understanding him was the key to beating him? In our game, Ireena succumbed to his emotional beatdowns and began to recreate history (the original Tatyana scene was about to play out). Understanding the bad guy should be a key to defeating him. Otherwise, it's just fluff.
The PCs figured this out after getting their arses handed to them and putting up a Leomund's Hut (I can't recall how they bought time to do that, maybe I was being generous). They thought: what would make a genius general give up his "can't lose" tactic? They'd seen Strahd get angry before, and it was only when bringing up his bad relationships. Their solution? Find Ireena, and stake her. They figured she's at the castle, and he's turning her into a vampire. They also figured there's a crypt with a ton of places to hide her. With nothing to lose (except their lives), they made a run for it, defending while they opened crypt doors until they found her. And they staked her. And Strahd lost it.
After that, with all the artifacts in hand, even beat up, Strahd was going to go down. It was satisfying for the players to see him go down quick. After all, he'd been beating up on them for the entire session.