That particular 'room' acquires a key for you to get there. How the key got with Tazok in the Windspear Hills in the first place is a mystery for itself
As there are Mind Flayers in the room, I would suggest using the Level 5 Cleric spell "Chaotic Commands" which protects you from their psionic attacks (and the Umber Hulk's confusing attacks). Use a lot of summons as well. Skeleton Warriors spring to mind, as they have a lot of resistances (magic, but also missiles!) so they could help you out. Use the summons to occupy them, while you sling bullets/throw darts/fire arrows until they're dead.
Buffing will of course also help (Stoneskin, Bless, Chant, Haste etc.)