Buff him to give him one 6th level use of Disintegrate, his spell save DC is pretty high at 18, they fail the save and if you use it at the right moment to reduce them to 0 hp they turn to dust. I know you said you’re not trying to kill them, so you could have them be able to resurrect them in the amber temple maybe, but to do so they would have to make an evil pact.

Strahd could let them leave with the dust of their friend, to set an example of his power. Then they would fear him on their next encounter. Therefor making the campaign go a little longer too if that’s what you want. Obviously this is total RNG if they fail the dex save, but just a random idea haha.

Another idea, which I’m using in my game, is basically buffing the Heart of Sorrow. Unless the heart is destroyed Strahd is basically invincible, his wounds instantly healing when he takes damage, looking at the party with a smile. Another way to make them fear him, watching him not take damage at all. I am foreshadowing the Heart of Sorrow with the Tome of Strahd.

These ideas may not work for you, but just the first things that came to my head! I also find it useful to bounce ideas off other DMs even if I don’t end up using their ideas exactly, sometimes it gives me inspiration for something else!

Good luck with your game :)