Source: Book of Spirits p137
The default dice pool for the Numina below is Power + Finesse, though there are exceptions, and a few Numina don’t require dice pools. Assume using a Numen is a standard action for a spirit.
When designing Numina, remember that contested effects tend to reflect all or nothing circumstances. The victim either becomes the spirit’s slave or not. Variable effects, on the other hand, are not usually contested. Normally, they are automatically resisted by subtracting one or more resistance traits from the spirit’s dice pool. The target can suffer none or some degree of the effect — such as being shot. The attack could be a graze, or a bullet through the eye. (Remember to describe it as such!)
Also when designing spirits or running them in a game, remember that spirits can put together enormous dice pools, just by spending Essence to boost their stats early in the combat scene. Spirits are loath to deplete their reserves, but if a Numen has no effect, a spirit can strengthen itself to take a direct hand.