That's a legendary boon, mostly because divine isn't appropriate for the primal nature of the boon. This is a flavor issue rather than a mechanical one, I think.
The boon itself seems completely reasonable to me. The power's fine as is. You could lose the Secondary Target line, which is extraneous, but it's clear what the power is meant to do and so on. If you wanted to root it in the primal aspect of the boon, you could say that the spirit companion needs to be adjacent to the enemy. Or perhaps adjacent to the ally? But the flavor text implies the former.
So I'd classify it as about level 3, give or take. It's more or less as good as a first level power, possibly even an at-will power. More flexibility is highly useful, thus I'd jack the level up to level 3.
I would make the acquisition of the power something that comes over time; consider the system for artifact concordance as a mechanical method. If the boon comes from a specific primal spirit, what does that spirit approve of? Reward that behavior with the boon. You could even make it more powerful as the character develops; it could be a daily power at first, and only grow into an encounter power. For a third stage of approval, perhaps you add a daily version of the power that does 2d8 or 3d8, with a bigger slide. The encounter power could also evolve into a 2d8 power without breaking the power curve.
When it's time for the boon to go away -- I think the prescribed period is five levels -- that might be the spirit telling the character they've grown and don't need that crutch any more. Something like, oh, "your allies have learned to trust you without my encouragement."