Memories of Teldrassil

Idriana was one of the bankers in The Temple Gardens.
Lariia was a Priest trainer in the Temple of the Moon.
Denatharion was a Druid trainer in the Cenarion Enclave.
Me'lynn was a Tailoring trainer in the Craftsmen's Terrace.
Dendrythis was the food and drink vendor that roamed the path between the Cenarion Enclave and the left side of the Craftsmen's Terrace.
Fyldan was the Cooking Supplies vendor in the Craftsmen's Terrace.

Once you're at Fyrakk's room, you need to send 6 people back down to the lower levels of the raid to collect the 6 Wisps. 3 are in the entrance area up to Igira's room and the other 3 are on the way to Council and Nymue. The first 3 are marked in this screenshot and the last 3 are marked in this one.

After finding them and clicking on them, the 6 players will get the buff [spell=431585] for 1 hour. This persists through death. Once 6 people have the buff you can pull the boss. It should be noted that for us when we wiped, one person (consistently the same one) was losing their buff and had to go back down to get the wisp again. We assume this is a bug but doesn't take long to correct.

As for the achievement, assuming you pulled with 6 wisps then in the second phase your adds will spawn one at a time with the batches of [npc=207800] adds you need to heal up. In addition to healing those you need to heal the big named mob before it reaches the heart. Once it's full health simply let it walk in. Most of the big adds don't spawn in the same location as the little adds, but will always spawn in the same order and location so it's at least predictable. I've included the order and location of their spawns in this raidplan and below.

1 - Idriana - left
2 - Lariia - right
3 - Denatharion - middle
4 - Me'lynn - middle
5 - Dendrythis - right
6 - Fyldan - left

You'll want to make sure to kill any of the [npc=214608] adds spawned by Fyrakk's breath before the adds reach them, and keep the infernals away from them as well. On most sets this won't be even a slight problem but on the very last add that spawns (#6 in the raid plan) spawns almost directly after the adds spawn and starts with very low health so it's very easy for him to walk onto an add and die. We countered this by Death Gripping that add off his spawn location but any knock would do or just nuking that farthest left add if you somehow have a comp with 0 knocks.

Once all 6 adds are healed up you can push the boss to last phase and kill him. If you fail to heal any of the adds up to full or if any of them die you'll have to die and try again.

If you need help with the metas feel free to add me to Btag (NA only) or Discord!
Btag - FrostyShot#1667
Discord - yarini

Our final wisp spawned right between two screamers and got turned instantly. We wiped and one of our people didn't have a wisp anymore. We (the one who lost the wisp and those of us who never had it) ran all through the raid and could not find a wisp to click. Turns out the wisps have a small respawn timer. I would say around 5-10 minutes. The one we saw respawn was the one towards the start of the raid and everyone could see it. So don't fret if your wisp dies. Just wipe and go get another wisp.