Casting the spirit rage spell on an ememy mage immediately provokes the nearest spirit into attacking them. From there onwards all normal conditions would apply as if they had attacked it first.
Only One spell at a time can have effect on any single mage so there would be no doubling up. If the spell is cast by multiple mages against a single enemy only the first one cast would count. All subsequent spells would fail until the spirit has been vanquished or the mage warps out of the spirits range and returns.

It's really simple, yet I think effective and could add a new dimention to both attack and defence.

Kinda like potting a catcher. Not unique enough.