Burning Bright

There are 4 Braziers in the 4 corners of the room and each needs to be hit by a specific ability to light them. You can see the Braziers marked in the screenshots here and here. I reference them here based on their name and their colour in my screenshots.

Fiery Brazier (Green marker) - Hit with Fiery Strike: Have one of the tanks pick up the Shade and run him to the Fiery Brazier and face him that way to hit it with Fiery Strikes.
Blazing Brazier (Red marker) - Hit with Blazing Surge: If doing the fight in Normal, Blazing Surge casts at the current tank, so have him face boss at the Blazing Brazier. If doing the fight on Heroic (or mythic for some reason) it can target anyone, so have your entire raid stack between the boss and the Brazier to light it.
Embered Brazier (Purple marker) - Hit with Ember Blast: Have your group get near the Embered Brazier and take and group soak an Ember Blast on top of it.
Smoldering Brazier (Blue marker) - Hit with Smoldering Remnants: Kite one of the Phoenix adds to the Smoldering Brazier and kill it on top of it.

We personally did all 4 Braziers in the first intermission, and we did Fiery > Blazing > Embered > Smoldering but you can do it in either intermission and any order you want. We also elected to add healers, both to push to intermission faster and provide safety with a prolonged intermission.

We pulled the Shade straight to Fiery, got it hit, then b-lined for Blazing and got that hit, at which point we dropped a Stampeding Roar to get to Embered and dropped the next Ember there. Then we kited along the entrance of the room to the Smoldering Brazier and started lowering the Pheonixes (we had 3 with 30 people). We were careful not to stack directly on blue right away, electing to drop the Ember Blasts off the Smoldering Brazier. Once the Phoenixes were low and close by, we used a Ring of Peace to keep them on the Brazier and killed them.

Once all 4 Braziers are lit, continue the fight as normal and kill the boss.

FrostyShot#1667 if you have any questions about the achievements! (US Servers)

Easy solo as of TWW lvl 80 and beyond:

1) Kill [npc=168973] and the other trash in the boss room.

2) [npc=165805] will come down from his pedestal and summon two phoenixes, don't kill any of them.

3) In the 4 corners of the boss room there are braziers, stand in them until they are lit by the boss' spells.

4) After all 4 are lit, kill the boss. (IAT can track if all 4 are lit)

Extra tip: bring some type of healing spell, potion or bandage. Some of the spells hit for hard still, and they circumvent shields.

This achievement can only be done during phase 2 (when the Shade of Kael'thasis active).

Facing the boss, we put the following markers in going counter clockwise:
Upper Left - Red
Back Left - Green
Back Right - Blue
Upper Right - Purple

We went counter clockwise lighting each brazier. Each brazier can only be lit by certain abilities.
Upper Left (red) - Lit by having [spell=326455] hit
Back Left (Green) - Lit by having [spell=328579] hit, these drop from [npc=168962], so the ranged who is targeted needs to kite to the brazier
Back Right - (Blue) - Lit by having [spell=325877] hit, this needs to be grouped soaked and is on a random raid member
Upper Right - (Purple) - Lit by having [spell=329509] hit. On normal this always goes on a tank, on Heroic/Mythic this is a random raid member

Light all four braziers, then phase the boss again. Finish the fight as normal.

Video guide: https://youtu.be/2vhRIhHJQqI