I found the earth spirit to be the hardest. I probably tried 20-some attempts at it before getting lucky and dodging just enough attacks to survive and get the final winning blow. I one shot water and fire, air took 10 or so tries. The pets i used for the various fights were: Bananas, Mini Thor, Fetish Shaman, Mirror Water Strider(i believe, the past one from the vale), Tiny Crimson Hatchling(or whatever its called from the cloud serpents), and the Highlands Turkey.
The dragonkin was for the air spirit, along with Bananas and the Water Strider. I used the Water Strider for every fight except the earth spirit, I ended up using Mini Thor, Bananas, and the Fetish Shaman for that one. The Turkey was used in the water spirit fight, I believe. And I believe I used Mini Thor, Fetish Shaman, and Water Strider for the fire spirit. At any rate, the earth and air were the toughest, but I chose the earth spirit from the quest because it was significantly worse for me. I don't want to repeat the experience of trying for that level of luck again! Lol.