Deck Reading Companion Card

I have a party with a cleric, a warlock, a paladin, and a rogue. They are experienced players (they managed to beat Lorgoth without anyone dying), despite my best efforts. So I’m wondering if anyone has recommendations for a good companion npc to give them to fight Strahd, or how much it matters with the current party comp?

(Also, somewhat selfishly, I don’t want them to be too unbeatable because I feel part of the charm in CoS is that folks die along the way, and idk if a powerful companion will help them ‘too much.’)