Deforestation (10 player)

Freya 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 or the encounter is reset. The Ancient Water Spirit, Storm Lasher and Snaplasher all spawn together, but only 1 of each per wave. To kill 2 of each, the first wave will have to be offtanked while two more waves (the single Ancient Conservator and the Detonating Lasher pack) are killed before a second set of Water Spirit, Storm Lasher and Snaplasher is spawned.

The adds in the Water Spirit pack have the following abilities:
Water Spirits - Tidal Wave
Storm Lasher - Stormbolt
Snaplasher - Hardened Bark

Especially the Hardened Bark ability is interesting, it's probably possible to keep the Storm Lasher rooted.

An entire pack must be killed within seconds of each other to prevent reviving all. This achievement also requires rapid killing of two packs, so that should not cause any issues. The exact amount of time that may pass between killing the first and the last of a pack is uncertain, but if it is very short and raid DPS is very high, it may be possible to use the adds from a single wave to get this achievement by killing them 1 by 1, allowing each add to revive before being killed again.

Note that killing Freya is entirely optional.

My raid and I got this tonight, more or less by accident.

On our third wave, we got the pack of three, which we began to dps down. Some mis-communication on our part led to one of the adds having about 70k hp left when the other two went down. We all jumped on the third add, but a split second before it died one of the other ones respawned, closely followed by the other one that had died. A few seconds later we got another wave, one of the big trees.

At this point we were expecting a wipe, but the tree add is significantly less deadly than the small exploding plants, so our tank picked up all four and we dps'ed down the tree. Moments after the tree died, we got a second wave of three adds. As one of our dps died, we really expected a wipe, but our offtank picked up the second wave, and our raid leader called for heroism. We began aoe'ing them down, and after about ten seconds they all died at the same time.

Ten achievement notifications went out over guild chat. We felt pretty pro.

Our raid leader said something afterwards that I agreed with: "You know, if we had tried to do that on purpose, it would've been a helluva lot harder."

I was very confused at start, but that's how I did this boss solo for the achievement:

1) I cleared the thrash and pulled the boss
2) killed all small adds or 1 big add that she spawned, left only Water Spirit, Storm Lasher and Snaplasher running after me
3) when it was about time for her to summon new ally-minions (dbm will show you), I killed 1 of 3 adds
4) she summoned 3 new adds while 2 from the previous summon were still running after me, the minion I killed got ress (took 12 secs), so overall I had 6 adds on me with the boss
5) aoe them down => achi