Deforester

Did this achievement in 2 last games (previous 3 I did different strategy with no luck). And can share my receipt of getting "Essence of Storms".
First of all - I dont think rares / bosses and other crap rly mean anything. Yes you can get buff from them by accident. But In my vision best way to do it is next 2 steps:
1) w8 when altar of storm will be completed. Before that you can do NOTHING.
2) when It completed you have to make a lot of killing blows of any type of creatures. So better choose packs with 10+ elves that spawn from base. Usually they have around 60k hp and one "fat". But you mustnt rly care about fat - just kill all "thin" as fast as you can. Better if you do this in 3 secs. Pretty ez with good gear. And go for next pack. Btw not all of them shown on map! So just camp "left" road (that close to water).

Following this way (kill as much as possible in 1 min) I got 2 essence in 2 games in 1st min after altar was completed. When you get ur monster from temple, better dispawn him imidiatly and run to next pack. I did this in 2nd game. Dispawned "ghost" (her damage is rly low. Probably because this is support unit. But who reads manuals? >.<) and run to my lovely shore described above. Ppl usually avoid this area so you can kill elves w/o "helping" of other ppl. Here I killed another 2 packs. When I w8ed 3d one I decided to kill some time and some deers as well. And got buff from first deer. So I can say next:
1) mass kiling blows most likely affect on process to become choosen
2) doesnt rly matter which npc do you kill. Most likely only counter mean. So choose targets with low hp.

In previous games I killed 2-3 rares every game. Did a tons of dps to fat mobs (who had 1000k+ hp), captured 4+ flags and had no luck. Also I tryed tact to kill elves at south. With same result. But when I started to do many killing blows this achieve become ends pretty quick for me. Maybe that was just random or "luck". I have no idea. I just share my experience and my vision of this situation.

Just got an essence after killing an angry treant that spawned, after I cut down a tree ... pretty much random I would say.

Edit: Getting an essence is most definately tied to killing blows. Did a few more warfronts and went out killhunting on normal mobs. Pretty quickly got an essence, choose the AzerMEK Prototype and just went ham on normal mobs. Pretty quickly a second essence dropped.

TL, DR: Killing blow on mobs gets you Essence of Storms ... happy killhunting

In the new Warfront, the Altar of Storms now gives you 4 choices, either to buff yourself, or hop in one of three "vehicles" which are the 3 needed for this achieve. Get the Altar of Storms built and then hope you get chosen for a buff. I had to do the Warfront 3 times, got one each time. Once in the vehicle you get 10 minutes, but can leave by right clicking your buff, without it affecting the achievement. Happy hunting.