Well let's see now. You harrass him and then he zaps you and you can't get through the portals with only part of your party!!! what might protect my mind long enough to get to the portals??
Yeah, the BG2 Flayers were much worse.
There are certain Clerical spells and maybe Wizard/Sorcerer spells that can protect your party or atleast one member of it while you try to kill it.