By Nature's Call

i can verify some things as off today for you from a alliance perspective:

1) Rares are irrelevant, i killed only a singular one (crab on the shore close to horde base) and got nothing from it.
2) All that matters is scoring a lot of kills, doing a lot of damage and tagging everything you can.
3) Assaulting mobs that run on paths towards your destination are a great method of singling yourself out from the group, so that some random spell caster doesnt get it instead of you, killing those alone i got my first essence.
4) Normal mobs can give you a essence as well, since i got my second one (within one warfront) from normal mobs spawned around the last camp, with the two huge robots guarding the last boss.
5) You can get another essence even while you are already transformed into a avatar.
?6) this i'm not 100% sure about, but there seem to be only ever 3 essences going off per fight, i always scan the groups debuffs and when i spot somebody else have one, i never recieve my third per battle. (this is edited later on)

tldr: Don't focus rares, quantity over quality, all mobs count.

In first warfront i got one essence from assaults, in second i got two as mentioned above and my achivement.

In third in which i wanted to cap my remaining two bases i got afked out and recieved a 30 minute deserter instead, because i wrote this instead of playing <3

Honey-Terokkar EU 5.1.19

Note that you can't get the buff until the Altar building is done, so there is no point farming rares and mobs at start.

1. Farm resources
2. Build Altar, but keep 40 resources
3. Buy the 40% damage buff
4. Kill the remaining rares
5. Push the opposite faction until the end
6. If you get the Essence, dont worry, you'll see it (entire screen becomes green and you get a message and a debuff)

Sadly, this warfront is quite easy so by the time you get the Altar half bases and rares will be done.

You can obtain this achievement by gaining Essence of Storm. You can do this simply by fighting mobs during the Warfront. As you do, you will not need to loot the currency; it is automatically added to your inventory (it is currency, it does not take up space). From there, take it back to Bashal'aran, and turn in there to either become the Faerie Dragon, Keeper of the Grove, or Dryad.

It's possible to get more than one per run. Your screen will blur with a greenish effect and you'll receive a debuff stating that you have been chosen.

Happy hunting!