Demidos has 2094K Hitpoints but is scaling with the number of people he is infight with. He does melee hits (average 11K on mail armor), uses Demi as AoE effect which deals 25% of players' current health as shadow damage and a single target Shadow Spike cast with ~12K shadow damage that applies a stacking debuff that reduces received healing by 5% per stack for 20sec.
In an earlier build we killed him with 3 people and it was really close. With his rare status we expected a guaranteed pet drop on the first kill or a drop for one group member but there was nothing but Apexis Crystals... so we wanted to wait for a new build to check for a changed loot situation. In the meantime other players reported pet drops on their first Demidos kills so we were hoping... but then Demidos disappeared somehow. Maybe he was always dead when I was checking his spot, but I wasn't the only person missing him so it was strange, I started to think he's only spawning with the Socrethar's Rise Assault daily active at the garrison but there's no connection.
Yesterday I went back to Socrethar's Rise and far sighted him but I was using the wrong client language so I had to prepare my action bars first to survive the hostile mobs on my way and then Demidos was already killed. I forgot to check the clock, but I think it was 90-150min until he respawned and then I managed to solo him with my enhancement shaman (635... T17 gear from vendor, Mythic SoO gear or new WoD items in all other slots, slow weapons, not that dagger shit from the premade I had to equip when we did it with 3 people). No pet drop... I soloed him a second time today... no pet drop. So I can't confirm a single pet drop in build 6.03.19085 (and for the records, it was the first kill on this character yesterday) and that's harsh for the difficulty of this rare.
Going into the fight...
I had a maximum of 7 stacks of Shadow Spike on me, so I expect this will go up to 10 (50% reduce) or maybe more. I didn't cast a single Lightning Bolt (that's a lot of damage missing) and used all my instants for healing spells, maybe that is something where I can shorten the fight a bit. I tried to interrupt or ground as much Shadow Spikes as possible and my elementals soaked a lot of the debuffs too so I usually had 0-20% reduced healing. I used every single damage cooldown, all my damage reduce abilities and it went a lot better compared to my first solo trys weeks ago. I haven't got a combat log or video of these trys so I can't tell if there was a nerf.
At some point in the fight I was low enough on Hitpoints so the Demi damage wasn't that critical with all the healing I did and this is something we should keep in mind... Demi itself hurts people but won't kill anything (okay following Blizzard's math it will kill us with 25% of 1 Hitpoint

I think Shadow Spike is not cast on a random attacker so it probably doesn't matter what's happening to the health of the players in your group as long as they don't pull aggro. Looking back to the 3men kill I think we wasted a lot of heal on the group because it scared us.
I don't know the numbers for different armor classes, but with heals (in your group) and several abilities to avoid Shadow Spike damage I think everyone can do fine. The shadow damage of the Spikes is predictable, so let your aggro-holder drop to a point were your healing is nearly equal to the incoming damage. I would suggest to use some of the "interrupt" abilities in a row to avoid the Shadow Spike and remove the debuff and not to try to slow the stacking by using them on cooldown.
So what I want to see are your experiences with this rare mob, your thoughts & suggestions and most important... informations about pet drops (first kill on that char, solo/group loot, respawn times, pet quality, breeds etc.). Thanks and good luck! @patf0rd