SPOOOIILLEEERRRRSSS*******************
I took my character level 16 players through tomb. A very large part of their success was first rescuing a scholar from White plume mountain to give them bit more information (They got a rhyme with hidden meanings for the spells that work on the demilich).
Second, one of the players was a bard, concentrating on knowledge (divine oracle prc). They used communes, divinations, auguries, and every other way to get information about things.
The druid used commune with nature to see whats under the tomb. At this point I made decision to only show the 3 initial tunnels. Everything else was in different dimension (the prison door being an 'invisible' portal too, giving only detection of magic, but no other visual effect).
I made it so that the different dimension was floating in some other matter and you couldnt destroy or alter the outer walls. Also, no teleportation worked.
This worked for the Maw at the start, for example. They also never used or touched the portals around. Few immune to acid and one immune to poison -persons helped a lot too.
They had a rogue with 50 max search (taking 20 + bard singing).
They searched EVERY 5x5x5 feet area taking at least 10. Sometimes 20. This took like few weeks of them, but food was not a problem. Myrlund spoon, few no food needed -ioun stones and Decanter of endless water helped there.
I kept the 1 soul/round, no save, no SR with the DL. In the end, 2 were in the gems, 1 was dead to the scream, one was ethereal (I made the spell do 25 damage instead of killing) and one (the melee guy who killed the DL in the end) did 1 damage /hit as he had disrupting weapon...
Lets say, 1 more hp for the DL and there would have been only mage left. Things got quite hairy this way too

. Happily, they had put some money for 3 True Resurrection scrolls before

. I let the corpses stay though.
If you have any interest, I could post my riddles for the tomb. I'm Finnish, so they are not excellent or anything, but they could give some ideas. Anyone who wants to see them?
SPOILERS END**********************