Each mechanic is simple. It is the combos that are difficult. The spell order does not vary in a phase. So we can learn the order and combinations of spells.
Bursting Dreadflame: Low damage AOE bomb around players. Avoid getting hit by more than one of these.
Focused Dreadflame (Beam): can be soaked in front of or behind target. We only need about 5 soakers in a 20 person raid. Do not soak if you would be within 5 yards of someone else. During Phase 1 and 2, soak line is between boss and Diamond (purple). Melee should avoid soaking. During Phase 3 there will be just one or two beams. The raid needs to adjust to soak since the boss will be moving around.
Phase 1 to 80%
Bursting then Armageddon
Singularity Corner then Beam
Armageddon + Beam
Singularity then Bursting
Phase 2 to 40%
Adds then Beam
Tank Add + Bursting then Armageddon
Beam then Singularity(*)
Bursting then Singularity in the Middle
Adds then Armageddon + Beam (*)
Bursting then Singularity then Armageddon (*)
Bursting then Tank Add + Beam (dps check)
Note(*): challenging combos the require high raid awareness
Fog of War + Adds
Phase 3 to 0%
Spells: Obelisks, Bursting, Beam, Orb, Gravity Rift, Darkness
The combos in this phase seem more obvious, however the boss needs to move a lot.
The raid needs to react to beam due to awkward positioning.
Drag boss away from orb spawn
Move boss close to rift to prepare for Darkness
Move boss to avoid orb fire patch
Lily and I made this spreadsheet to give another view of the timeline. Note that the timings are approximate and depend on how long it takes to get to 80% and 40%.