Took 20 hours or so as a 209 ilvl Kyrian Discipline Priest full PVP gear.
I think 200 ilvl or more is recommended to kill layer 8. RNG inside can make it ez mode or not. Disc priest is really cheesy and has some nice powers but it's very sloooow.
Kill conditions: Get Catharstick and/or Scales of Trauma in combination with Schism (25% more dmg) and powers that make mobs take extra dmg when feared (35% additive). Run in schism, fear, nuke.
Soak dmg to refill Catharstick and Trauma for Elites/bosses. These two go up as your HP goes up so stack HP powers as well. Average hp per run was 170-180k. Last run was 380k LOL
Survivability: Volatile phantasm + Fragments of Obscurity for big AOE and dodge death layer 6+ (floor 17/18)
Any fear increase/duration/damage taken are good as well.
Don't underestimate Psychic Wallet. At least 1 is recommended to gather a lot of phantasma while MC-ing mobs.
More helpful info: here.
Good luck!