Now that is just lame
BGT gives you the "Helm of Sarevok" which makes you immune to INT drain. It was the new "Helm of Munchkin" for me as combined with Lilarcor (charm immunities) and the ring of free action it enabled Minsc to clear out Illithid city alone without taking more damage than the ring of regeneration would heal during the fight.
So while I find immunity to INT drain itself overpowered as it take s the Flayer out of mind Flayer "Would you
Mind if I ravage your city? HAHA", putting INT-drain immunity on Lilarcor (the obvious choice against MFs before aquiring the Psionic Blade) is just hyper-powergame-munchkin. All you need in addition is a ring of free action and even if you don't have that the Flayers will only do 2-3dmg/round which a tank can easily take for the duration of a stun or hold person.