The reason I suggest overmatching your characters is that horror is not about a fair fight or balanced encounters. The big boss fight should be something that makes fleeing the sensible thing to do.
I was in my regular 5e game a couple of weeks ago and the party was facing some ghouls and ghasts. Three of the ghasts were massive and super tough. One character got two attacks per round with natural weapons. The big bad ghast steamed into the battle and took our best fighter down to half hit points in a single round. Then this character had its go, with Action Surge going it was up to four attacks. The player rolled a 19 and three 20s. The critical gave them another action point and they got another natural twenty on one of those attacks. The ghast that we thought was going to be the death of us was torn apart in 6 seconds.
While taking out the ‘big bad’ in a single round is cool, it is not what horror is about.
Cthulhu games are built around threats that cannot be defeated, just staving them off is the best one can hope for. That is the core of the horror.
Putting an American Werewolf in London is scarier than putting it in a dungeon. Your typical London police officer has a truncheon and a taser, neither of which are silver or magic. The werewolf in the dungeon would probably quake in fear at the approach of the typical adventuring party, loaded down as they are with spells and magic weapons.
King Kong wasn’t a fair fight, he could take out aircraft with machine guns!
You can use terrain to put the odds in the bad guy’s favor, or superior knowledge, like Minotaur in the labyrinth. Or just pick something with really tough stats.
Running away is always an option, until the villain has you cornered. But that is part of what horror is all about.
These tips are taken from my Scarier Horror Play book. You can get 40% off this book at DriveThruRPG using this discount link. You can get 40% off all new releases as a loyalty discount if you join my contact list.