Getting Back to Nature (25 player)

Freya begins the fight with 150 stacks of the Attuned to Nature buff. Each stack increases healing done to Freya by 4% (= 600% at the start of the fight). She keeps healing herself throughout the fight.
She also summons adds in three different compositions. The initial order of add summons is random, but once three different packs have been summoned, that order will be repeated until Freya is killed, the encounter is reset or all 6 waves have been summoned. Killing adds removes a specific amount of Attuned to Nature stacks:

Ancient Conservator - Single add, killing it removes 25 stacks.

Ancient Water Spirit + Storm Lasher + Snaplasher - Three adds that must be killed in a 10 second time window to prevent reviving all. Killing each removes 10 stacks (=30 stacks total).

Detonating Lashers - 10 Spawned per wave. Killing each removes 2 stacks (=20 stacks total).

To get this achievement, she must have 25 or more stacks (=100% or more bonus healing recieved). Killing all 6 waves exactly removes all 150 stacks. Freya will have to be killed before she looses 125 stacks, so she must be killed before the second Ancient Conservator or Spirit/Lasher pack is killed. This pack will have to be off-tanked until freya is killed.

we just got the achievement while freya had 30 stacks, so it only requires her to have AT LEAST 25 stacks, not exactly 25

That makes sense considering if she has 30 stacks, she has 25.