Deforestation

In order to obtain this achievement, you need to kill 2 sets of Water Spirits, Storm Lashers and Snaplashers within 10 seconds in phase 1. Wait for all of them to spawn, AoE them down and then defeat Freya.

Easy enough. Aggro Freya and let her smack you for a few minutes. She will spawn 1 Water Spirit, 1 Storm Lasher and 1 Snaplasher in a group, at a time. Took me maybe 3.5 minutes for the 2 groups to spawn. The achievement did seem to take around 4 seconds to actually appear.

I can confirm you have to wait around 3-5 minutes for her to spawn a second wave of Water spirit, Storm Lasher and Snaplasher. Once she does, you can freely aoe all the mobs down including Freya. Don't panic if the achievement doesn't show up immediately! As Callmejoe finely added;

Callmejoe 2018/07/28 "The achievement did seem to take around 4 seconds to actually appear."

You will not get the achievement before the boss fight is declared completed, so just wait :D