Yes, its true, Mind Flayers were created by Satan and somehow the BG Team felt too evil to get rid of them (similar to level drain and Beholders). This is how to kill 'em.
1) Scan your area...Wizard Eye, thieves, invisibilty.
2) Try to draw some of them out one at a time. Get a range attacker...shoot 'em...run. The Flayer will follow. Make sure you have as many remove paralysis spells as possible. Get 'em read if a Flayer 'holds'
3) OK, they are quite weak...kill them quick though. They rely off gimicks to kill. That is hold, domination, stun and intelligence drain.
4) If a character gets INT. drained... get them out of the killing area immediately. They will recover after not too long. However, if they are INT drained then it only takes a couple more hits from a Flayer to kill em out right.
5) Have patience and quick save after EVERY kill. And don't be afraid to rest...it isn't the weaklings method! Remember: fail to prepare, prepare to fail.
Good Luck