You should go back and read the part on his tactics.
Strahd fights dirty. The players should feel like the fight is unfair because it is.
Strahd has been aware of the PCs since they entered Ravenloft. He has been scrying on them and knows their weaknesses and strengths.
He attacks from the most advantageous moment and position. This means he will attack at the end of an arduous adventuring day, perhaps while the PCs are currently engaged in a different fight and after he has charmed a PC (see below).
He knows when he is in over his head. This means that he will use hit and run tactics as needed. He could come in and attack for just 1 round at a time if he wants.
He uses his charm ability on the PC with the weakest saving throw. This is perhaps his most potent ability. Do they have the Sunsword? He could get a charmed character to get rid of it for him. Etc.
He can also cast 5th level spells. Animate Objects, Greater Invisibility, Polymorph, Fireball, etc. (For example, imagine the PCs are resting, Strahd beats their lookout's perception because he is invisible and can float through walls and such. Then for his fist action he casts a 10d6 fireball on them. Each PC takes 35/17 damage then he is gone again, and this is after a long day and they still haven't gotten a long rest. At any time he can come back and do it again for 9d6.)
This is why Strahd is CR 15.
This is super misleading to new dungeon masters. Why?
Since you are effectively
describing something cool, not
explaining it. In other words, you leave everything about HOW to make this happen up to the individual dungeon master.
A new DM needs much more than "find out the PC's weaknesses and strengths". A well-built party of heroes will, to the bad guys, appear almost invincible, with many more strengths than weaknesses.
Hit and run tactics sound like a cool concept, but in the framework of 5E, it simply doesn't work. As a delaying tactic, yes, but as a way to whittle player characters down? Come on. We all know the only way to beat super-resilient foes (and PCs are among the most resilient creatures in all of D&D) is to use overwhelming force and keep doing your maximum until your enemies are all dead.
But you know what? Fighting dirty (as in frustrating any counter attacks) is not fun. Is Strahd really supposed to have to use cowardly tactics to threaten the heroes? Hell no!
What is "an advantageous position"? You or I might know. But you aren't talking to me, you're clearly addressing a new DM.
But all you're doing is making them feel bad for failing, like it somehow was their fault.
In reality, "CR 15" is meaningless. If you just run a CR 15 monster straight off the book, a party will demolish it at level ten. And indeed, they might well succeed even at level 6.
This is the real problem.
The fact a new DM will probably fail at running it "right" suggests there is somebody else to blame here. Not the DM. The designers and the adventure module, for not reality checking the fact their precious CR 15 won't stand a chance against a well-built party of level 6, let alone level 10.
The fact you
can make the monster work does not change the fact that if you use it in a straight-forward manner, it doesn't.
I'm sick and tired of people defending 5E monsters (and the old chestnut "even kobolds are deadly if only used right"). All it does is it lets 5E get away with being so weak on monster design. It shifts attention away from the real question: "why aren't monsters given enough oomph to truly challenge player characters right out the box?"
Then new DMs would have the tools they need with no extra effort. Even I sometimes just need a monster, but I don't have the time or the inclination to play it "right". I just want to play it "stock".
And as you gain experience and confidence, you can start using lower-CR foes when you intend to run them "right".
So, please, stop suggesting CR 15 is somehow mighty if you only "run it right". It shifts the blame away from the game and the adventure design, onto the hapless DM's that don't deserve any.
Strahd's stat block should have been designed to present a serious challenge for a CR 10 party in a straight-up fight. Ideally, a borderline unwinnable one. And there should have been more story threads as to why Strahd would allow defeated PCs to live (to try again). Which is exactly why you would want to find the items and the allies the adventure provides.
Reading about yet another DM having his BBEG be shredded by low level PCs is so... weaksauce.
Reading posts that tell DMs that BBEG just needs to be played "right" is so... disappointing.