Karamor is 100% correct. Consistently, the CoS groups aren't optimally structured to deal with Hard/Deadly encounters. This because they have a single point of failure for healing, or dealing with downed characters.
Take the goo that was hitting like a truck. Let's say it deals 25 and downs a character. A single level of Cleric/Druid/Bard allows Healing Word - as a Bonus Action (so your "main function" continues) - to stand up the character that was just splattered. Who cares if it's inefficient? The game says - negatives don't matter all that much unless it's going to outright kill you. So, even at a terrible efficiency - say 1 HP - you get that person back up.
The goo swings again at the character. 25 points. Down. Stand them back up with Healing Word for 1. Goo swings. 25 points. Character down.
Here's the thing. That's 50(!) points of damage that other characters didn't take, and 2 rounds of Goo barshing. That's the difference between a TPK and an encounter you can demolish.
Too often Berg lives, because he's useless at just sending over a single HP of help, and then he runs.
There's enough god-meddling in CoS that almost any character can justify Cleric or Druid at a single level. And if you start at 5? 4/1 with the one being a class that can help in times of high stress makes sense.
In the group I run, a group of 5, has a Cleric/Druid/Paladin/Sorcerer/Rogue. They are really, really hard to kill. The Protection Paladin with the cheesy 1 HP Lay on Hands makes Deadly single monster encounters almost Trivial. This is the game part of DnD that CoS characters haven't quite figured out.
Adam, rightly, kills characters with encounters that "shouldn't be deadly" but that's because he plays them optimally and for keeps. He's playing the game part of DnD fully. If the characters don't play for keeps and are structured sub-optimally, deaths will continue to happen.