It must work different for tanks then. Every single time I've tried to use the Special Action button for Fading Light, once I come out Ultraxion has gone Immune and is casting his raid wipe. I thought it was that you can't have the entire raid out of the Twilight Realm, but after looking it up, I believe that's just for when he's casting Hour of Twilight.
As for beating Ultraxion, even at 585, there are times where he just blitz's my Voidwalker and its game over. Usually beating him for me goes hand in hand with how long my Tank is up. If I can keep my first Voidwalker up until after the 1st Hour of Twilight, it's usually a kill. The fight definitely has an element of RNG to it.