It was actually possible but pretty difficult and required a lot of luck - you just had to hope that he didn't target your demos at all. Even if a demo was targeted early you'd be fine, since a competent driver could keep the pyrium or w/e debuff rolling while kiting the boss, just isn't possible later on. If he targeted sieges and bikes, the demos could put out enough damage to take him down before everyone else died (though most would).