Off Balance

Considering the problem of Serpent valley I came to the conclusion that it’s an issue of abilities.
It’s hard to counter a spellcaster at the best of times but when a Party of six contains four of them, any hope of control is gone.
We’ve effectively got four characters that can fly, turn invisible, inflict massive damage from great distances, escape from most traps and rest in absolutely safety .
This leaves Durin and Thesis very square pegs in a Sorcerer shaped Party.
What can be done to fix the situation?
Nothing really.
It’s not right that the DM changes the world to beat a Party that has adapted itself to conquer it.
The Giant snakes would have been a real problem to the standard Party of Fighter, Rogue, Cleric and Wizard, but our Party's numbers and wizardry made them irrelevant.
You didn't even have to bother facing them.
They were (I suppose appropriately considering they were snakes) beneath you.
Saying that, the final encounter will pit the Party against the most powerful creatures in the D&D Universe…
Adventurers!