Cast cloudkill into the room they are in (with an improved invisibled character) then shut the door. The cloudkill will kill the Umberhulks pretty quickly, and damage the mind flayers some. Then you can either repeat, or you can charge in with your hasted tanks to make minced (mindflayer) meat
.
To protect your characters, either cast chaotic commands or have them drink a potion of clarity or a brine potion if you have any. Or have them wear/use any item that grants protection from charm, confusion etc. Sending in summoned critters will work too, especially invisible stalkers.
Oh, and don't forget to save each time you clear a room.
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"Butt-kicking for goodness!" - Minsc
"Cities always teem with evil and decay. Let's give it a good shake and SEE WHAT FALLS OUT!!" - Minsc
