Obviously Tons of Spoilers below..
The biggest thing you should realize is that if the party is formidable, Strahd excels at hit and run tactics.
Notes:
Strahd should start without his link to the Heart of Sorrow. Strahd has regenerate 20, every turn his heart takes damage instead of him, its wasting this powerful ability, he can reactivate it as a bonus action if he gets low on health and has to continue fighting.
Lair actions.
Passwall - The most important one is the fact he can pass through floors and walls as if they aren't there. Combining with his legendary action to move, he should never be stuck when in danger, he can sink a floor down even for a couple of turns and recover health.
Closing/locking doors - This is very powerful for splitting a party if you convince them to give chase to you. Even if they can bust the door open, it uses up an action.
Summon Spectre - This is really weak in combat, however, this is an amazing harass ability Strahd can use while hidden to torment the heroes (He has a +14 stealth skill).
Steal Shadow - Again, not super powerful in combat, but he could use this against his opponents while hidden, and instruct their shadows to keep following the heroes. (Maybe allow an insight/perception check to notice the players shadows are acting unnaturally.) once combat does start, there's a few more targets around.
Legendary actions.
Move - Remember this works with passwall, if you get hit for a lot, at the end of the turn just sink through the floor
Unarmed Strike - Decent damage, and possibility of control, probably the action that will be used the most.
Bite - Really should only be used if you need health and need to continue the fight, it deals less damage than unarmed strike and uses 2 actions.
Legendary Saves.
These should really only be used when it has potential for extreme danger. Turn undead for example will break when he takes damage, allowing him to use a legendary action to escape at the end of that turn (if necessary).
Spells.
Animate objects. If the party doesn't have strong AoE. This is a very powerful spell, dealing potentially 10d4 for 10 turns.
Polymorph - Obviously powerful for either disabling an opponent, or turning himself into a T-Rex with a new HP pool, and as importantly, losing his vulnerability to sunlight for the duration.
Greater Invis - obviously a powerful offense and defensive ability,
Fireball - Again, so obviously powerful with hit and run tactics.
Mirror Image - This is very powerful as it doesn't require concentration, thematically I might run it as him shifting out of the way of an attack at the last moment due to his inhuman speed. This is another option he may cast on himself just before approaching the party, or whilst on the ceiling below the party for a round regenerating.
Darkness - This is not on his spell list. BUT It is a wizard spell that thematically fits with him. Its power is multi fold. It suppresses the light of the sunsword (Darkness can only be bypassed by SPELLS, the sword isn't a spell) So that he can fight without worry. Also two creatures that can't see each other have advantage AND disadvantage which means it negates the disadvantage he would have because of the Protection from evil spell on players from the Icon of Ravenloft. Also since he can have it cast on an object, it can be in his pocket and removed when needed or put away when convenient as an item interaction (basically 1/turn free action)
Allies.
Don't forget that Strahd has allies at almost all times. A few dire wolves or ghouls may not sound like much, but they can distract and otherwise harass the heroes.
Creatures of the night probably should be used before he actually engages the players(stealth +14 remember) as its a pretty crappy use of an action in combat.
CRAZYTOWN: Have Strahd begin the fight wearing his animated armor. Having 112 temporary hit points, armor 21, adding 1d6 lightning damage on melee attacks. This is obviously stretching the rules a bit, could allow for an interesting fight as he would have 2 distinct phases, Fighting as a bruiser hitting everyone at first, and afterwards using deception and skirmishing tactics. (I would probably have him phase walk out of his armor like he does through walls once it is destroyed).
Basically remember that Strahd is a regenerating wizard with powerful melee attacks. He is not a bruiser that should go toe to toe every round until he or the Heroes dies.
Finally, remember that even if they players do defeat him, Rahadin is supposed to attack within moments. So keep in mind that before this, Rahadin should never overtly attack the players as his goal is to allow his master to do as he pleases. Rahadin himself is fairly illusive as he has misty step and a high stealth skill as well. He has potential to deal up to 46 damage a turn, so could potentially finish off an already weakened party.
