If going by the DA:Vampire rules, which also apply in DA:Werewolf by the way and so are not actually crossover rules at all as they are in Vampire Core basically, then you roll at normal difficulty for the damage but the Spirit may soak with it's Rage. And on the other end of the spectrum the Spirit also uses it's Rage as effective Strength and so deals damage accordingly, again using normal difficulty.
If you mean in modern setting then by WtA revised you would roll damage against the spirit at the difficulty of it's Rage. However you might wish to wait for the final judgement on the matter to be resolved when W20 comes along, which will be soon I hope.
Unfortunately V20 took a bit of a backwards step on this matter with Rego Elementum 5 Summon Elemental using normal stats to describe the Spirit, or perhaps a step forwards in the unlikely case that W20 Spirits will also have normal stats, instead of Rage, Gnosis, Willpower and Essence division used by DA:Vampire.
это на тему духа.... ну в принципе вампиры тогда то же элементалей призывают как тут было замечено)