Helior - 10 man
FIRE ELEMENTAL
lvl 255 (or 500) (essentially hidden level but lvl 255 so we can actually get hit)
4,300,000 hp
4500-5500 FIRE damage
2.5 atk speed
30% damage reduction
Immune to Fire

Living Bomb (yes, the one from Geddon)
Rain of Fire (from Gehennas)
AOE Fear
Enrages after 5 minutes - adding A LOT of damage (I recommend Battleguard Sartura's hard enrage)

I was hoping a boss like this would be found in Hellfire somewhere

I assume by 10 man u mean 10 man instance? or maybe a measure of how many needed to kill it? otherwise sounds good.

Measure of people needed to kill, with a dedicated tank so a advanced AI.

Thats my though