dionim shared a video of him defeating the encounter. You can find some tips here on Reddit.
Whitefriday shared some helpful tips for defeating the encounter.
Save your cooldowns and a Starsurge proc for the first transition. Kite the boss around the outside to avoid dropping purple puddles in the center. Use Starfall primarily to get casts off. The transition to the next phase takes place at 33%.
Be ready to go hard on the boss immediately when he becomes targetable. Use drums, cooldowns, a second potion, and burn the boss hard. Adds will die naturally in the next phase, so ignore them for now.
You need to keep slight pressure up on the boss, but your main concern is killing the hands quickly. Kite the boss with Starfall as you did in Phase One. Maintain as much Astral Power as you can without capping and save Starsurge procs. Spend excess Astral Power on Starfall when you aren't busy killing hands and continue casting on the boss. When a hand spawns, Typhoon and root the boss. Spend all of your Astral Power on killing the hand with Starsurge or Starfall if the boss gets too close. If you know you won't be able to kill one fast enough, Solar Beam at the last second and let your DoTs do damage as you continue on the boss. If a cast is about to go off, use Bear Form and Barkskin, topping yourself off with an Empowered Regrowth (Legendary Boots). The transition takes place at 33%.
Burn Raest and ignore the adds. Don't miss runes. You must be within the rune as it expires to prevent an add from spawning. If an add does spawn then it is a wipe. Stay on the move with Starfall and plan your route so that you aren't caught standing in a rune longer than necessary. You should constantly be on the move, jumping into runes right before they expire to avoid the boss and adds.
Karam re-enters the fight, but ignore him and the adds. You want to kill Raest quickly while managing hand damage and runes efficiently. Handle the runes and hands that same as you did in previous phases. Keep moving and use cooldowns again as soon as you can. If a rune spawns and Raest is almost dead, you can ignore it and focus the boss. Once Raest dies the quest is complete, so there is no need to kill Karam.
Writhing shared some helpful tips for defeating the encounter.
Talents - Malefic Grasp / Contagion / Circle / Sow the Seeds / Burning Rush / GoSup / Soul Conduit
Pet - Doomguard
Legendaries - Sacrolash's Dark Strike is pretty much mandatory for this fight. I can only assume the purple puddles on the floor were meant to slow the boss, and they are potentially bugged. As a result, the slow from the ring is necessary unless you can DPS fast enough while kiting perfectly with a combination of Gateway, Demonic Circle, and Burning Rush.
Kite the boss using Burning Rush while maintaining the Corruption slow from Sacrolash's ring. Should the boss melee you, he will one shot you as his stacks get progressively higher. You may also make use of your Doomguard's cripple here to further slow the melee boss.
When the melee boss reaches ~30%, he will become immune and begin regenerating health. During this time, you are able to attack Raest. I suggest using your potion, Drums of Fury, and saving both Soul Shards and Reap stacks for this burst phase. Additionally, three waves of 4 shadow fiends will spawn which you should Seed of Corruption (which will slow them) and use the Soul Flame damage to continue bursting Raest.
When the melee boss returns to full HP, Raest will become immune, and the melee add will begin chasing you again. During this phase there are two extra mechanics in addition to the original phase 1. The first is a portal which MUST be interrupted by your Doomguard and nuked before you are overwhelmed by additional portals or the melee add's speed stacks. I've been informed that these portals can be banished, but I certainly wouldn't take the chance with being overwhelmed by multiple of them being up at once. The second, is a rune on the floor that you must stand on and soak as it expires, otherwise it will spawn a Faceless add which will most certainly one shot you.
Once the melee add reaches 30%, you will be able to attack Raest again for the remainder of the fight. You will get quest completion upon killing Raest, although I did end up killing both in the end. I suggest nuking Raest while continuing to kite the remaining adds and nuking any portals which spawn.
Killem shared a video and some tips.