I was stuck here too.
This is what worked for me, although it's probably not the intended path to take.
Use a Avatar of Storms scroll on a character to get him immune to the stunning cloud. Have him flip the switch (it doesn't switch off the sentinels, but it does, I believe allow the door to open).
Get another character that can turn invisible close to the final two sentinels (but so that they have not spotted him; the door should then be open and there should be no lava). Have him turn invisible and simply walk past the sentinels and into the next area. There'll be another switch there that does take care of the sentinels.
