If you have Mazzy in your party, it ain't that hard.
Pump her AC and THACO down as much as you can, and equip her with Sword of Arvoreen and Shield of Harmony, and if you have them the Boots of Speed and the Girdle of Hill Giant Strength.
Have her run in the sight of one flayer at a time, and let it chase her down the corridor, where you can easily take it on, with the help of some archers.
Mazzy's sword grants protection from Stun, which is invaluable aginst flayers, and the Shield takes care of the rest. The low AC is needed to prevent INT drain.
Keldorn is also great for fighting illithid, but you still need to prevent the Stun somehow.
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