This is still bugged on Test? If so, that's disappointing because it means another disconnect between what the developers think should be happening vs what actually is.
Their regen is bugged, yes 110 should be able to roll over all the fabled in PoTime, but as a 115 nearly maxed AA Beastlord I can get them down to 90% burning them down but they'll just regen to full HP after one tick sadly don't have assasinate/Headshot so can't even try to burn them down without other being present who can single target fast burn, so unless you have a group of super high Burst damage there is no way anyone is getting them down before they regen full.
This also happens on other Fabled bos encounters. On Antonius Bayle, The Fabled Terris Thules from her Lair and Fabled Saryrn from PoTorment also regens to full every tick. And I heard people saying the same about the Manaetic Behemoth from PoI as well. The only way to kill them, is to burst them down in one tick which requires a force far beyond what is expected for these bosses or a huge burst spell aka Mana Burn/Harm Touch/etc. I hope this can be fixed too cause it prevents many things from happening in the game. (flag progression, achievements, loots,...)
So it looks like its been fixed as of the live patch now.. That being said, any chance you'll extend these fabled for a week or so due to the woopsie that happened and we missed a whole month of being able to raid these! (PoTime raids was our guilds big thing, the fabled gear was well sought but unfortunately, we only get one crack at it this year )
