I did this elemental tonight with my SK 82 (plus the pet) and a healer merc (j2)and my mage 72. I needed the epic click and two of my veteran powers, but still had deflection and lifeleech back up my sleeve. The elemental conned yellow, so 85 is probably correct. The elemental spawned in the last room, when the mage entered the corridor towards it. This section is of the halls is not on the map I had, as someone posted below (go towards HoH B and turn left there, and go until the last room). It was in the lefthand corner area when you enter. Fighting it there is at least away from the ***** mobs near the door.
I could not find a way to get rid of his DoT (justice), which does about 260 per tick, which lasts for almost 5 minutes, but it is overall only a nuisance. It hits for 518/1691, usually at the low end, but occasionally it does a flurry where it reaches 2,2k per hit. It does hit pretty often, probably since it is higher than me. Initially I went sword & shield, which I quickly abandoned in favor of 2H DPS. After a few rounds of careful watching I was sure that I gain more from using the 2H than from the shield defense. Since it has only the single target DoT as special attack (you can neglect the other effect, which it does once every 10 minutes), this is just a tank contest. I discount its rampage, as it did no real hurt. It does have a lot of HP, but it might be less than 1.2 millions, as the HT did make a visible dent into it.
Notes:
- the mobs around there are a nuisance, grey as they may be, as they all spawn two adds after death
- I started with the healer merc on passive and activated it only when I needed it; the elemental does resist lifetap spells, so I did a lot (he ended up OOM, but than it was no j5). I had a second healer merc on the mage, also kept passive to replace the first, should he go OOM.