Like xymox, ranged and heals stack one step behind boss in order to bait out purple sleep puddles (identical to the current dread lord sleep puddles in M+ this season), these puddles move so melee need to be mindful of it. Tanks will be proactive about moving boss to handle puddles.
The fight has three phases and is a standard council style fight. During the majority of the fight, we deal with sleep puddles, an add, and a green split circle/purple purge circle mechanic. The add is summoned periodically at 1 hp and casts a spell repeatedly. Each cast heals the add and it cannot be damaged until it has reached full health once. We let the add heal up to full HP, then maintain interrupts and focus down the add.
Some players will be marked with green split circles and receive a stacking DoT. If another player is in your circle range, the DoT and circle passes from you to them and you're left with a debuff that makes you take more damage from future circles for a period--avoid passing the circle as much as possible. Melee that get the circle should immediately pass it off to a range, and ranged should handle stacks as needed, passing if it's unmanageable (we'll figure out the magic number). On each second cast of green circles, two players will be marked with big purple circles. Purple circles need to run out from melee range and stay put, ideally not overlapping. Anyone with a green circle needs to stand on the edge of the circle, careful not to pass the green circle to the person with purple or anyone else, and wait for the purple to expire. When it expires, the green is purged and releases slow moving slimes that will apply green circle to anyone they touch. There is a weak aura that shows you the orientation of the slimes, so use that and aim them in such a way as to maximize room for melee. If you don't have a green circle, don't go in the purple or you'll be feared. During the second mini-part, the bosses turn into bugs and pulse big raidwide but also take increased AoE damage. We lust up, pop personals, and slam damage here.
During the third part, the bosses vanish any adds and circles remain in play and we become hostile to each other. Two players will be imposters and a handful of us will be able to spot the imposters. Those who can spot the imposters will see floating dreadlords above the imposters, and will use a Weakaura to vote those players as imposters. The imposters will also see those same demons over non-imposters. To counter this, we wait for someone to reach 3 votes and then "kill" them to remove the dread lord--they will be knocked back when "killed" and then any additional damage will actually kill them, so swap off accordingly.
This entire fight is largely about managing circles and adds--slimes must be dodged at all costs and purges cannot be missed.