[Faceman]

Greenstone Amulet and Boots of Speed.
Well this is a cheesy tactic, but it's the easiest.
Conjure up a Mordenkaine's sword and kill it.
It will leave a Greenstone amulet that equips you with many immunities including immunity to psionic attacks. This buggy amulet cannot be removed IIRC and will fizzle if you use its abilities from the item abilities menu.
With this amulet and the ranged weapon of your choice just step into the room and keep running in circles shooting the Flayers. Never engage in Melee!

Another - not SO cheesy tactic: Use the character with the best saving throws (or equipped with anti-psionic items) to lure the flayers out of their room one by one and await them with loaded guns (i.e. The whole party firing their ranged weapons at them) Their psionic blast will stun one or two party members, but the Flayer will most likely be dead before he can kill them.