Just to understand this:
You want to possess the earth. With a fire spirit?

May I direct you to the following text regarding inanimate vessels:
"The spirit can use any of its powers through the vessel itself, but it can only move the vessel in ways the vessel can normally perform." p.197
That seems to be the last place a mobile and active fire spirit would ever possess
You would have trapped your spirit in one patch of ground - ground Kynos just drove over at considerable speed (4 = 40m/turn) with lugged tires.
Sounds to me like the spirit would have to resist Body (=5) AP 6 damage.
Just for the record, I don't like this use of possession in this kind. In my opinion you need a discreet object as a vessel not a patch of ground (especially not one where living gras grows and is part of the gaia sphere). And unless you spread that spirit thin like a pancake (which I don't think is possible - you can't possess just part of an object) you would have an unmovable patch of earth where you couldn't possibly predict the course of the bike and still be ready to do your action. At the very least I'd have expected the spirit to make an edge check to get his ambush site right.
Possession is itself a complex action and until Kynos attacked you did not know where he would come from. So the spirit would have to choose his spot beforehand while Kynos fired the first grenade and beat Kynos' Ini of 30 to do his attack.
I won't contest your ruling, but it takes a lot of fun away for me if any planing I do is countered by ready enemies that don't have to deal with combat distances and time constraints - my only advantages while I'm dealing with mojo-slingers.
Next time it's simply grenades upon grenades with a side order of grenades until no NPCs are alive to give me grieve
