Hows a group supposed to get past area 20 if they don't happened to have a wizard with disentigrate spell there seems to be no other way to progress deeper into the dungeon.
They're better off that way. At/near the end are some monsters with DR like 50/adamantine or something, and SR much too high to beat at that level-- so if you don't have a lot of tough fighters with adamantine weapons, you're quite doomed. "Mud Sorcerers Tomb" was seemingly designed to one-up the original "Tomb of Horrors;" it's a classic DM vs. players TPK scenario.
Hows a group supposed to get past area 20 if they don't happened to have a wizard with disentigrate spell there seems to be no other way to progress deeper into the dungeon.
Stone Shape or Transmute Rock to Mud on the walls; a magical item that can be found earlier grants Transmute Rock to Mud if no one in the party can cast is naturally. Or just smash an opening around the Wall of Force effect; as a "classic" dungeon, the only time limit the party faces is food supplies (although a cleric with Create Food and Water renders that to "when do I die of old age?"), so even time-intensive "solutions" like this are feasible.
the stone shape and other spells from the ring probably wont work cause the stone had been altered to be as tough as adamantine but i tought about it later and realized that a wind walk or gaseous form could get them through the peephole