[PseudoFenton]

Originally Posted by Kuroshima

Of course, I do have two glass cannons and two turtle tanks in the group, so the sigil would be more like turtles with glass cannons on them. Probably laser cannons ;)

Nice, I can picture it now! My group refer to themselves as the "Strays", or "Kaylees Strays" now (after an NPC they all hang out with), so their version would likely have cats instead of turtles.

Omission, {snip} It doesn't breathe

Cool cool, I assumed as much.

Those are underwater volcanic vents, anyone passing through them takes 3d burning damage. {snip} The characters are protected, and so is the boss, but I keep the hazards so they realize that the cash spent on the protection was worth it.

Ah, that wasn't noted anywhere. Not all groups are so well provisioned, especially when their GM refuses to acknowledge those amulets as cheap forms of immunity, making this a little more hazardous to them.

I assume the sea-elves don't have the faculties to swim around these regions either? Making clever positioning and herding an easier method to rid yourself of them before they become snack-food?

Indeed, only corrosion heals. The DR is external, {snip} Getting explosive inside will be very effective

Very good, the multipliers for internal explosive injury was what I was betting on here.

It's possible to enter. {snip}
Leaving them open will cause a tasty sea elf to wander out each turn, but that might end up being counterproductive as it will mean that Igghagh will grow much bigger. If it comes to it, I'll roll 6d×10 for the number of sea elves. A much better strategy might be to seal them, either with control gate or with shape earth, to restrict Igghagh from healing.

Shiney, I'm pretty sure this would be the first thing asked about/attempted if I ran this. Glad to know they'd be even easier to slaughter as they don't put up a fight even in that pocket dimension.
As for opening the rift being counter-productive, perhaps not. It's HP has a cap, but I'm assuming it just keeps eating, meaning you can deplete its store of snacks before opening up with the big guns (rather than dividing firepower and/or fighting against the attrition of it healing). The fact that you know it wont have access to accelerated healing also means you can more easily plink away at it over time, even with resting/retreats.

The other option is to use the stream of sea-elves as a distraction or lure. Keeping it in once place whilst you rig the roof to blow, crushing it under tones of rock, say.

I'm not complaining, though my players might ;) It helps me be better prepared for playing this on the table and not get blindsided by my crafty players.

Well they might not try such obtuse tactics, especially when half of these tactics are being asked because I can see the mechanics behind the scenes - so perhaps its not overly useful.