Or if you have no Pro. from Evil spells or scrolls, send the demon in and close the door
First time around I killed Flayers by getting as many summons as possible and standing them just infront of a door, blocking anyone going in, or out. Then I pummelled them with arrows, it seemed to work.
Second time around I used my ranger (wearing leathers, has Hide in Shadows) to sneak until one mind flayer could see him. Then when they would cast the stun spell on me I'd run back to where the rest of my party were (I had boots of speed). Then, before the spell caught up, I'd give Imoen the boots of speed, and send her off to watch the Mind Flayer come. Then I'd get everyone to attack it, it might get the occasional Int. Drain but no one was badly injured [img]smile.gif[/img]