User blog comment:B214/Question and Answers 4/@comment-24412159-20150220134931/@comment-1359322-20150220140206

Actually no, the Phenex's immortality doesn't make them immune to Holy Water and Light, they are still susceptible to those. The only advantage of the Phenex's regenerative ability is that they can restore limbs and heal instantly from any minor injuries, aside from that, they are still open to severe damage and the normal weakness of the Devils.