...okay so this is gonna get stupid, and I don't have the actual building fully planned out yet but here's the concept:
The stronghold part is extremely simple. There is a deep pit with a building over it. At the bottom of the pit is a portal with a trapdoor over it. The building is essentially a small warehouse that serves as a breeding ground for a very specific kind of ooze. This is an ochre jelly that has both the Sentry Ooze and Mineral Warrior templates. Such a creature has been purchased from Neogi slave-traders, who would've had to track down or breed a very specific variant for this customer, and thus it is 4 times more expensive than usual for its CR: a total of 25600 gp, which cannot be reduced unless I'm gonna become a slime breeder.
These slimes have Int 1, and are capable of taking up to 3 feats. Those feats will be Final Strike, Improved Unarmed Strike, and Versatile Unarmed Strike. I can use magic to change whatever feats the one I purchased already had into these feats for pretty cheap, I imagine. For the next part I'll need some method of commanding/controlling/training the slimes, which should be doable.
There are two spots in the warehouse that are like...resetting healing traps, or maybe wondrous architecture granting fast healing to one specific square. In one of these spots is the ooze originally purchased (81 HP). It changes its unarmed strike to slashing, hits itself, takes no damage, and splits. Now there are two ochre jellies, each with 40 HP, each in a healing spot. Assuming this is like...resetting cure critical trap, that's 4d8+7 HP per turn, average 25. Any split oozes that aren't healing will help continue splitting until they can't split anymore (10 HP or less) and then they'll go crawl down the whole. I think they can run with a climb speed, but I'm not 100% sure.
Spoiler: Process
Show
Round 1
(1) 81 HP
Round 2
(2) 65 HP
Round 3
(2) 57 HP
(2) 32 HP
Round 4
(2) 53 HP
(2) 28 HP
(4) 16 HP
Round 5
(2) 51 HP
(2) 26 HP
(4) 14 HP
(8) 8 HP
Pit +8 oozes
Round 6
(2) 50 HP
(2) 25 HP
(4) 13 HP
(8) 7 HP
Pit +8 oozes
Round 7
(2) 50 HP
(2) 25 HP
(4) 12 HP
(8) 6 HP
Pit +8 oozes
Round 8
(2) 50 HP
(2) 25 HP
(4) 12 HP
(8) 6 HP
Pit +8 oozes
The part of the pit the slimes are filling up is 572 feet deep. Ochre jellies are 6 inches tall and so this fits 1144 of them in the pit, a vertical column of ooze. Going from a single ooze at full HP to 1144 oozes available for the pit takes about 147 rounds; with a bit of wiggle room added to allow those later ones to move into position, that's a ~15 minute recharge time, potentially longer or shorter depending on specific rolls for the resetting Cure Critical trap (although with how many casts there are, I'm assuming average will be pretty close). The last 4 ft are the trapdoor and the portal; the other end of this one-way portal is attached to the bottom of the Bus.
The bus positions itself at least 200 ft above a given target. The trapdoor is triggered (maybe remotely, maybe by taking the two-way portal back to base to pull a lever or something). The trapdoor pulls away, and 1144 oozes fall 576 feet in the first round. This has the last of them just barely touching the portal and going through, leaving an empty pit and a once-more-closed trapdoor over the portal. The oozes will be somewhat aimed based on Aerial Bombardment rules (HoB pg 67-68). This will involve an attack roll (at -4 penalty from normal, -2 extra for every 50 ft above target, presumably with no maximum range since it counts as thrown but talks about attacks aimed more than 250 ft down) against AC 5. Presumably, if we're 200 ft above target and only taking -12, that'll be a breeze for any nigh-epic character.
The oozes fall, and land. whatever was in the 10x10 area they landed on takes 20d6 damage, as does the ooze itself. Since the ooze has 10 HP at most, 20d6 kills it 100% of the time, which triggers Final Strike. This is a 60 ft radius explosion with a primary 6d6 bludgeoning damage (Ref DC 21 for half) and an Earthquake effect, as the spell, presumably only affecting the 60 ft radius. After that, the body is so destroyed that it takes True Rez, Wish, or Miracle to bring them back (so effectively they've been disintegrated, nothing remains behind). In the first round, 744 of the jellies will make impact; the other 400 instances will hit during the second round, giving you time to escape if you somehow barely survived.
1144 instances of 20d6/6d6/earthquake is going to wreck basically anything it hits in a 15 ft diameter circle (the diameter of an ochre jelly), and wreck structures in a 135 ft diameter. You drop that onto solid stone, and there's a 15 ft diameter hole going straight down 476 ft, with everything within 60 ft of the hole severely damaged by the massive number of earthquakes that just occurred. if you dropped it on a structure, then each of those jellies is also dealing 100 damage (bypassing hardness) to every structure within 60 ft of the impact zone. If you did that to the Bus while the prismatic screen was down, the Bus would be destroyed in just 6 hits.