The Immerseus solo is theoretically doable for any range class with a pet and able to avoid damage/heal themselves.
I managed to get it down to around 40% in total before I lost concentration and fucked up on my Frost Mage (with only 548 ilvl)
However, yes. I does take a very Very long time (took my about 90mins to get it down to 40mins)
ps. The people that say pets take 90% less damage from aoe are correct. However, they take 90% less damage from Unavoidable aoe. They take 100% less damage from Avoidable aoe. As all the damage apart from the explosions when the boss remerges is avoidable then the pets take no damage at all apart from that 1 point where you can very easily heal them back up.