What the achievement asks
Complete a Superbloom at epic bloom quality. Five points.
Blizzard ties this one to the character, not the account, so a Superbloom run on an alt does not close it for your main.
The Superbloom is the large event in the Emerald Dream. Its final reward comes in a satchel whose quality depends on how well the run went, and this achievement wants the top tier.
Quality is about effort, not crowd size
The instinct is to find the biggest group possible, and it is the wrong instinct. Players in enormous groups tearing through the route report barely reaching blue quality, while a player doing the entire Superbloom on their own reached epic comfortably.
The reason is that bloom quality is driven by the bonus you build up from the things you personally do along the route, and in a large group there is nothing left to do: the mulch piles are already mulched, the flowers already tended, the primalists already dead before you arrive.
What to actually do
Take part in everything, not just the fighting. The route is lined with small activities, and each one you complete adds a bonus on top of the bloom it gives.
Tending flowers is the one to focus on. Each is worth roughly five bloom, and a solo run stacking them produced a bonus of around forty-five on its own, which is what carries a run from rare to epic.
Druids have an easier time here because their travel form makes covering the flowers between combat points quick. It is not a requirement, only a reason the reports you find are often from druids.
So: go alone, or go with a small group and spread out rather than clumping. Do every mulch pile and every watering you pass, kill the Feral and Stalking spawns you find on the way, and do not stand around waiting for the front of a zerg to finish something for you.
Where it fits
Part of the Emerald Dream branch of A World Awoken. The same run also feeds the other Superbloom objectives in that branch, so a single well-played event closes more than this one.