A Fungus Among Us

Easy Achievement for Battle for Azeroth Level 120, and beyond...

1) Normal Difficulty. Grab the flamethrower, do not use it yet.

2) Begin fight, reduce Boss Health to around 40% or so.

3) Let the green moss cover most of the room so you have plenty of it. May take a few minutes.

4) Walk forward into the green moss and tap the flamethrower button.

5) Keep tapping it and walking forward into the moss until you get those 15 stacks of Burning Infusion, I did 16 just to be safe.

6) Nuke boss.

7) Profit. Achievement Completed.

6.2 update: This achievment is not personal anymore

Several tips and information that you may find useful:

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• Like some people said, it's a personal achievement. It can be done in normal and heroic (2 flamethrowers) or mythic (3 flamethrowers) difficulties.
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• You don't need to have the stacks up at the end of the boss. E.g.: you can get 15 stacks in the middle of the fight, and then finish the boss with 0 stacks.
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• Creeping Moss grows faster as the boss is lower, so it's better to wait until the boss is below 40-50% like Exorsist said.
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• Use Escape key to cancel the flamethrower channeling, it's safer than clicking on the ExtraActionButton1 again (if you click twice, you may activate the flamethrower again). You can press Escape several times to ensure you cancel the channeling.
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• Wait until you have 0-1 overheated before trying to get 15 stacks.
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• We found better to set fire on groups of three squares of moss with short channelings. Go to a corner of moss, face one side, activate flamethrower, rotate 90º, and cancel flamethrower. Go fast to the next corner of moss and repeat.
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• After getting 15 stacks, start doing the boss as normal.
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Faster ways to get the achievement for your entire raid:

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• You can die and still get the achievement, so after you reach 15 stacks you can kill yourself in order to give the flamethrower to another person and get more achievements per kill.
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• Also remember you can repeat the same boss as many times as you wish during the same week in normal and heroic difficulties (just make sure you have a saved char in front of Brackenspore for quick runs).
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Things we've not tried:

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• Dying after getting 15 stacks to let someone else get 15 stacks with the same flamethrower. If it works, it may help to get the achievement for a full raid in less weeks.
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I hope this helps you. Good luck with the achievement!

PS: Edited with the new information. Thanks! ^^