Open Door, fire wand of cloudkill, close door
That should take car of the Umber Hulks and if you get stunned you can just wait for it to wear off.
Then try the following
- move in with one buffed up tank (Ring of Free Action, Charm Protection Items, Boots of Speed,...) and employ hit and run tactics either with a ranged weapon or genuine beat-and-run
- gather your party in the next room. Approach so that you're only attacked by one of the flayers and lure them back to your party who is waiting with every available ranged weapon. This tactic again requires the Boots of Speed and Ring of Free Action