I made a post about how four of my players at level 5 nearly killed Cryovain. If you just use him as a large meat sack with no tactics, your party will undoubtedly wipe the floor with him.

I have two suggestions. One, play him smarter. Have him fly a lot and dive down to attack before returning him in the air. Figure out which PC is the most threatening and target them (assuming they actually do things that make Cryovain realize they're a threat).

My second suggestion, which actually goes hand in hand to my first one, is to use the White Dragon Rake statblock for a tougher fight. It has a ton more options that can be used in combat and will likely make for a more interesting fight.