Praise the Sunflower

To get this achievement you will first have to make sure that at least one player has the [npc=51090] pet.

The achievement is personal, and requires the player to use a /praise command on a [npc=51090] pet, while under the effect of a [spell=284802]. The pet doesn’t need to be your own.

To get under the [spell=284802], one or more players should pick up [spell=284645]. This jewel will grant the player a stack of [spell=284664] while they are stood alone. Upon reaching 100 stacks, they will gain the [spell=284798] aura for 20 seconds, granting the player and all nearby players [spell=284802], increasing critical strike chance by 100%. The buff will take about 100 seconds to appear, and once it's active /praise the [npc=51090] pet to get credit for this achievement.

You can set the [npc=51090] pet as your focus target and then use this macro so you can quickly praise it:

/tar focus
/praise

Note: The pet cannot be targeted by /tar commands or macro arguments, that's why the macro is built this way.

Update for TWW, for those who are, like me, wildly confused by these comments because they have never "properly" fought Opulence:

Note: If you want to solo it, you'll need a pet class

1) Get the Singing Sunflower battle pet from the quest Lawn of the Dead (Go to the Wowhead page for the quest if you need help beating it)

2) Summon the Sunflower before starting the fight.

3) Remove your gear, summon your pet, and have it attack Opulence before it aggros on you (if you only aggro the boss with your pet, the boss eventually targets you again and resets the encounter)

4) Now that the boss is fighting your pet go to the left/west corridor. Once in the big room, go to the next hallway and you should already see interactable items. Click on the Topaz of Brilliant Sunlight to get the buff you need.

5) Walk back to the boss(no need to hurry) and wait for your Brilliant Aura buff to reach 100 stacks (you might want to bring healing of some sort and dodge the gold pools, they can still do quite a lot of damage on Level 80!). Once that happens, you'll gain the "Grossly Incadescent!" buff and you can now /praise your Sunflower pet.

6) Kill the boss and enjoy your achievement :)

I hope this helped someone! I was quite confused by these comments because everyone who says you need a pet class doesn't really emphasize the fact that your pet has to be actually attacking the boss for it to not despawn, so the encounter kept resetting for me. After figuring it out, the achievement is quite simple really :)

Note that players need to remain stacked on the person carrying the [spell=284798] aura and then /praise the Singing sunflower. You will lose your buff if you are too far away from that person.

Players that are playing on a different language need to /praise in their language.
I made a list of the /praise command in every language

French: /louange
German: /preisen
Spanish: /alabar
Russian: /похвала
Italian: /elogiato
Portuguese: /elogiar