SPOILERS
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1. If you have some 'collared' mindflayers, you should only have to move one near the door for it to spring open
2. I hate mindflayers. If you engage them in melee combat, they will suck your brains out of your skull. The higher your intelligence, the higher you last (ie. never let Minsc anywhere near them). So you have 2 courses of action.
a) stay as far away from them as possible. Raise undead is the best for this if you have a cleric in your party, as undead have no brains so are immune to brain sucking attacks. Failing that, try any summon monsters spell, just to get a wall of flesh between your brains and the flayers tentacles. Then take pot shots from a distance.
b) use potions of intelligence, and send your best (preferably most intelligent) fighter in to do the deed, backed up with ranged attacks from the rest of the party.
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Time is an illusion. Lunchtime doubly so.
Epona of The Laughing Hyenas.