Want to make Ravenloft really difficult for your players?
Give Strahd some help, in the form of a high level cleric with a new spell, called Resurrection of the Undead as Undead. Then take that cleric and hide him/her/it (as Strahd would do) in some remote place well away from the Castle. Or even go so far as to put that cleric in stasis, with a contingency set up that if Strahd is killed, the stasis ends and the cleric wakes up, geas/quested (you just can't trust evil clerics) to raise Strahd.
And if you are feeling really nasty, when the party returns victoriously home from killing Strahd, have an angry Strahd send them a little 'victory present' to show him his appreciation of their efforts to kill him (make sure it hurts enough to make them want to go back after him again, of course.)
I suggest - just a suggestion - that you strictly stay away from the old cliche of: party finds casket, party opens casket and finds sleeping vampire, party stakes vampire, vampire dies. This is an insult to Strahd's intelligence (not to mention Dracula's.)
If your character had a special weakness, wouldn't he take countermeasures to protect himself? Sure he would! So, why wouldn't Strahd. Certainly, he didn't get to be Count Von Strahd, ruler of Barovia, by being silly or careless.
If your players have a problem with this, remind them of all the extra experience points and/or treasure they'll get, killed Strahd the second time around. And remind them that they chose to take on this powerful, dangerous opponent. If they expect you to be a nasty DM, then be a nasty DM and remind them that - until Strahd is gone for good - they are marked men and women for the rest of their lives (and unlives, which won't be very nice as Strahd exacts a cruel retaliation.)

Greetings, Cam. Cheers to Dragonlance. It was my favorite main setting.
If you are REALLY feeling nasty, have a kender somehow sneak along with the party into Barovia. This shouldn't be so hard, considering kender (if one of them could sneak into Par-Salian's Time Travel spell, one should be able to sneak into Barovia with adventurers!)
Then, once the kender is Barovia, naturally he or she will want to:
1. Explore the Castle (neat Castle, heard all about it!)
2. Meet and chat with the Count (Vampire? He must have some interesting stuff to relate!)
3. Learn all about undeath (just how does that work? Is it magic? Is it something else?)
4. Learn if there are any kender undead.
5. Ask if the Mists contact Krynn, and where, and how.
6. Ask how Paladine and Takhisis think of Ravenloft and it's interaction with Krynn.
7. Ask questions all hours of the day and night, ESPECIALLY when monsters are approaching that the party is trying to ambush (let's go talk to them!)
Remember, players, that killing a kender is an evil act. Restraining a kender is nearly impossible. Leaving a kender behind in Barovia is tantamount to murder, and also thus an evil act. And committing evil acts in RAVENLOFT ... is a *real* bad idea ...
If nothing else, your players will have more sympathy for the Heroes of the Lance after they survive the game ...