Deforestation (25 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.

Just did this as a 652 fury warrior - literally sat there and waited for 2 sets of the storm/snap/water to spawn. The only thing I was killing was the trees she spawns, and that was only b/c I couldn't remember if they made the adds immune or something.

As soon as the 2nd set spawned, I waited a second for the water mob to come back to me (he charges around randomly), popped recklessness, ravager, and bladestormed them all down. Turned on Freya, burned her and profit! :)

Easy Mode for 110

Step 1) Prepare - Dismiss pets, make sure you don't have any gear that auto-attacks or auto summons anything. Hunters, wait till freaking Hati despawns, you don't want him killing the boss and ending this quick.

Step 2) Engage - Ride your mount up to Freya, sit under her for a couple seconds, she'll start the fight. DO NOT ATTACK. Don't worry about clearing anything, at 110 they can't hurt you or do anything of note. Everything in the room will come at you, but you are impervious to their ill intent.

Step 3) Wait - You can basically go do whatever you want for awhile, as you wait for there to be enough adds to get the achievement. I went to use the bathroom, then I headed to get a bowl of cereal. It was a tough decision between Cinnamon Toast Crunch and Apple Jacks, but I went with the former. Sat down, ate my cereal, watched my character getting kicked around (but staying at max health). I waited till the enrage timer was at 3 minutes just to be sure there were enough adds of each type.

Step 4) BRING THE PAIN! - Unload whatever quickest AOE you got, for me, of course, multi-shot. Keep hammering till everything is gone. Be sure to spin around in case anything is behind you.

Step 5) Enjoy your achievement.

Cliff Notes - Start fight, sit there without touching the boss for 5 minutes, then kill everything really fast.