As far as i know, the sixth or seventh time you'll get a warning, and the time after that you'll fight the Big Cahuna...
He isn't much better, by the way. Just some higher level spells than the others - but if he won't have the chance to cast those anyway, it matters not.
All are very vulnerable to Insect Plague, Traps, Skull Traps, etc... The usual Mage slaying tactics.