Alright, this is an update from this post here
I know @NerdyAarakocra and @Cash67 wanted to see it, let's @Voidus too, just in case
I finally finished with the encounter.
Yes, I know it's been four months, but schedualling is hard.
So I decided to make the encounter a little bit harder, and split it up into multiple parts, first the party would be ambushed by multiple kobolds, a dretch, and if they stumbled into one of the traps a blight(which they did, and ended up with a vine blight on their hands). They were then able to make it too the fortress where three kobolds with crossbows stood on top of a wall and fired down on them, the door was locked and they had to break in.
I kid you not they took a grand total of 0 damage by the time they finished with all of that.
Then, came the kill chamber, fifty feet by five the entire thing covered in oil, caltrops and ball bearings, ten kobolds lining the hallway, behind arrow slits, with three flying kobolds above behind murder holes, and a fighter in plate with a shield the polearm master and sentinel feats, and a Warlock with Repelling blast at the end of the hallway.
The fighter kobold starts the round off by lighting the hallway on fire, which will burn for 2 rounds dealing five fire damage any time a creature entered or started it's turn in the flames.
Things go wrong instantly. The barbarian rips a door off the front and holds it in front of him to push the ball bearings and caltrops out of the way. He has an AC of 32, and enough movement to just run down the hall, turning the entire encounter into a joke. I have only three chances, the flying kobolds above have nets, if I can manage to hit him with one I can halt his movement.
And I did, I managed to crit on the last possible moment, and prevent him from clearing more than three squares.
After that fire and eldritch blast took care of the door.
And despite all of that the players still moved through the fight. Turns out the lowest AC was around 17, and they all had +8 or higher to hit. The fighter kobold dies almost instantly, and the warlock has to run away, left with only a few hit points. The rest of the kobolds die over the next couple of rounds, but are fortunately able to deal a decent amount of damage before they die.
The players break out into the courtyard and face down a minotaur skeleton that I gave the maximum number of hit points to, with two gargoyles, and two more kobolds coming in for assistance.
Finally, after I dealt three crits in the courtyard alone, I manage to down two players, but by that point all the enemies have been defeated.
All in all I learned some very valuable lessons.
Raw stats alone can effectively make players several levels higher than what they should be on papper
Turns out that having a tank that can't be hit by anything short of a crit, with +10 to all saving throws makes it really easy to tank unreasonable amounts of damage.
I'm still stunned that they were able to take this, and am going to have to think up some other horrible things to do in the future, but I say it was definately worth it.