Due to low ceilings, hiding inside alcoves near the skill point prevents players from getting thrown up in the air by the vortices. (Make sure to fully stay inside an alcove and then just dodge/walk away from the ice storms.)
Standing on a vortex deals around 1k damage and applies 1 stack of vulnerability, which however, is still a better deal than instant death.
Vortices can overlap. In the worst case I’ve ever experienced, it’s 3k damage plus 3 stacks of vulnerability, which is still better than getting dropped 3 times in a roll to an instant death even with the trait of 50% falling damage reduction.
Vortices do not appear all at once. They gradually appear one wave after another in a random pattern within 1500 units around Grenth. So, instead of risking dodging from one vortex into another, hide in an alcove and stand on them!
Hiding in an alcove by the skill point also line-of-sights Grenth away from stairs. (To some extent, because Pact NPCs fighting Acolytes, spiders, or wraiths, have a chace of taking aggro away from players and causes Grenth to move to the stairs.)
Most of the time, shades and mobs are aggroed by Pact NPCs running towards Grenth. However, by the time the last wave of shades spawns from the right room at 25% of Grenth’s HP, Pact NPCs could not get into the temple due to increased mobs’ activities in the tunnels.
The last wave of shades is usually what causes a lot of deaths for a low DPS group. The number of shades scales from 4 to 8 or even more, when 5 or more players are participating in this event. With the normal rate of corruptions stacking every 3 sec with 1 shade up, 8 shades mean 12 sec till 25 stacks of corruption, i.e. the final moment that instantly downs a corrupted player no matter what.
So, if all players are hiding in alcoves by the skill point, there is a good chance that the last wave of shades will all stay in the right room without getting aggroed.
Hence, hiding in an alcove by the skill point solves all 3 common problems – vortices, Grenth’s position, and last wave of shades.
The downside of hiding in an alcove is it’s harder for ranged players to strafe/walk away from Grenth’s slow-travelling frost bolts, which hit the current aggro holder and 2 other randomly-chosen unlucky targets.
Another interesting fact is that vortices have a hidden multiplier that increases falling damage. Even without the falling damage reduction trait, a normal fall from that height can not kill a player with full HP. I’ve personally tested this by jumping off of various places of this height in this room after getting tossed up and using air-control to land in a holding cell, on a rafter, on a scaffold, and on top of a broken pillar.
This tip comes from the experience of a bomb/grenade engineer who has successfuly soloed this priest 3 times with hundreds or maybe even more failed attempts.
TL/DR – Hide in an alcove by the skill point.