This appears to be a scenario written to annihilate any party other than a fully optimized party that spends lavishly on items that would never be used in any other scenario. Note, not just kill the PCs but annihilate and reduce them to unresurrectable characters without items.
As I read the reviews, I can see that parties which spent lavishly and selected niche magic items (at this level they are indeed niche) to counter a very specific threat were able to defeat the first encounter with luck. Normal parties have no chance given the sheer number of saving throws every round. Especially if you are playing hard mode which has a good chance of killing a PC a round. It is not a question of tactics but rather if you make the saves since missing almost any 1 save will make you NPC chow.
I played in the 7-8 tier with 4 players and there it is basically a toss up whether you will face a CR 13 or only a CR 11 encounter right off the bat
This scenario left such a foul taste in my mouth that I actually wrote a review