[Faceman]

well though there are enought flayer threads out there here are some tactics
for easy/cheesy FlayerSlayers

lure them out of their room and into yours one by one (run away and close the door behind the first that comes in)
Attack with ranged weapons while one of your chars is running in circles away from the Flayer who is attacking him.
Use all brute force you have on one Flayer to make him go down before he starts brain sucking.
Immediately Remove Paralysis if someone gets stunned (else => brain suck => dead)
backstab with thief can get you a normal flayer in one hit!
Do NOT try to use magic on them as they have HIGH resistance but use magic to buff your party - chaotic commands, haste, bless, chant, ...
After a MF fight take a break and wait for the braindrain to wear off. Go to the fridge and get yourself a soda or do anything of the like. Remember that energy drain and intelligence drain wear off after a few rounds, only level drain lasts until dispelled.

NOW for the mega cheese:

pre tactics mod MFs are stupid so the classic cheese is to have an invisible char standing in the door while you attack with ranged weapons.