One other thing of note than most seem to neglect. Tweak the stat black within reason if you wish. First? Get rid of the stupid ruling of him not turning into a cloud of he dies in sunlight or from radiant damage. The game assumes the party has a sunblade and at least one character suitable to wield it. Furthermore, strahd is no more vampire, it would make sense he functions differently.
As those have said before, use the environment to his advantage. Cast fog cloud to even the odds with attacking to negate the sunlight penalty, have him sneak up and charm the fighter/barbarian before combat even begins and have them turn on the party, have him run as he fights, leading the party into other monsters, traps, and away from his coffin. The castle is his domain, use it. Additionally, don't forget that he has 50 additional health from the heart of sorrow in the castle tower.
If that isn't enough? Consider giving him more health if you've got a party of munchkins. I actually made it so that he literally could not die while the heart of sorrow beats, and that both he and the heart could only be reduced to 1 hp unless the heart was destroyed directly in the tower. It should be guarded VERY well too, and is conveniently in the complete opposite end of the castle from his tomb.
Want to be especially cruel? Charm the person with the sunblade and order them to throw the blade off the cliffside or the castle, or have strahd's minions move the coffin of the players find it before strahd. If Rahadin is still alive he most certainly will be ruthlessly defending his master, as would all of strahd's consorts.
