Yeah, with the artifacts in hand, I think he is meant to be a pushover once cornered. The issue is cornering him or getting him to fight fair. Over the editions I've run encounters with him with most characters being 5th-8th level. Always using hit and run tactics. He's only been defeated twice, out of about eight different groups. First group was in 2E, a group of 7 being 10th level armed with the Sun Sword and eventually cornering him in his crypt during the day. Second group was in 3E, a group of 6 being 14th level and one of the PCs having been a werewolf to boot on top of his character levels.
So far, I haven't had a party defeat him in 5E, though I've only run it twice as a one-shot only using the original town of Barovia and the castle (but the 5E adventure). Both times, the party had badly injured him and were in pursuit, and he used the castle to lure them into traps before counterstriking, with deadly effect. I pulled no punches, used his abilities to full effect and doing my best to emulate his genius-level intellect with metaknowledge and his shapeshift ability to have already spied upon and size up the party.
In short, I don't fight fair with him, just shy of cheating, and treat him as I would if he were actually my character trying his damn best to wear down (mentally and physically) the PCs, and destroy them at his leisure. He's immortal with hundreds of years behind him and ahead of him - he can afford to be patient. He manipulates events to his advantage, not the other way around. If the deck is stacked against him, he gets out of Dodge, and simply waits until he has the upper hand to return.