Freya starts out with 150 stacks of Attuned to Nature. Killing each wave of adds removes stacks from the buff, so you'll need to plan on holding off one set of mobs in order to get this achievement. Killing an Ancient Conservator (big trees) will remove 30 stacks, killing the Ancient Water Spirit, Storm Lasher and Snaplasher group will take off 30 stacks, and Detonating Lashers will remove 20 stacks. The easiest way would probably be to have your OT hold the second Ancient Conservator that spawns while the group kills off all the other waves. You will then need to kill Freya while he holds off the big tree. This way the only thing you need to worry about it standing under the mushrooms vs. worrying about charging Water Spirits or Lashers detonating.