I've run this thing four times now, and seen it run twice by other DMs, and never once have I seen it play out the same.
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Thus far, I've seen four different strategies attempted for breaking down that sealed door, all of which managed it within two rounds, even though three of those four attempts relied heavily on luck.
I'm glad you're having fun.
Me, I wouldn't want to try to solve the same problem of getting through the same door over and over again. I'd want to try the other routes. To me, going through the same door over and over again IS running the encounter the same way over and over again. The minor details on individual tactics and results would be different, but the idea is the same, hence, it's not very interesting.
If the encounter allowed the DM to switch up the floor plan, to have the floor runes do good things some times, bad things other times, and random things some times, that would be enjoyable. If the mud room pillars raised up and down in one encounter, but forced the PCs to always go down in another encounter, or different sets of pillars were up and/or down each time with potential places to attach ropes, but also ways for those ropes to break, that would be interesting. If the cloaks helped some times, but set off a trap other times, that would be fun.
If the encounter allowed the DM to do fire damage some times, acid damage another time, blinds some times, weakens another, stuns another (note: these PC debilitating type effects that should be rare in a normal game should be common in a DM killer dungeon encounter), and a wide variety of effect, that would be cool.

I guess I just appreciate more imaginative encounter design then what was supplied. I appreciate a design that also makes the game new and different for the DM each time beyond which monster and trap he puts in which room.
Every time in the dungeon would be totally different for the players, not just a little different, and hence, a lot more interesting than going the same route, kicking down the same door, and facing the same BBEG with slightly different monsters in the way each time. The floor plan and the obstacles are too static and predictable, and there are too few monsters per room, for this to be a real challenge more than once. The entire encounter is focused on damage and except for the BBEG's immobilization, there is little in the way of riders. To me, that's a bit boring and not much of a challenge cause damage can be negated and healed easily.
And some of the awards are kind of lame too. I really could care less about creating a party where everyone is the same race. That doesn't really make the encounter more challenging. It makes designing the PCs more challenging. Eating the fish once means that any PC with a high Nature roll allows the PCs to eat the fish every time. That's not a challenge, that's a gimme in future runs through it. Different stroke for different folks.
I'm glad that it works for your groups. It doesn't work for me.