Word up Front
This zone is the stepping stool from Anguish and/to the Daily Hardcore Planes, to Crystallos.
That simply means, that if you cannot yet properly handle the Planes, (zone bosses excluded), you are most likely not capable of handling this zone either.
Recommended for this zone, maybe not so much for trash, but definitely for the bosses, is around 135K AC, and 900+K HP.
Your DPS is recommended to be at 90k+AC, and about 800k+ AC so they are better able to withstand AE spells and damage.
This also goes for ranged, as some mobs will leash after a certain short distance, and if ranged is too close, they get killed by the AE.
You can obtain the quest by talking to the NPC from the picture, in Barren.
Phinigel isn't overly difficult, recommended you use levitate or Deadman Floating on whole road. You WILL be flung across the room to dragons that are hanging from the rafters. Diablo has fewer hp's than the rest, hasn't done anything special of note. Vaniki's guards need to be killed in the lava pit before engaging him, they will come to his aid and they hit about as hard as he does. Otherwise tank and spank.