Like many things in DFRPG, opening the barrier can have many causes. It could be a simple act of will with no spellcasting involved (Fae in natural areas, WCVs in a strip club, etc). It could also be a quick evocation or, for longer and more extensive portals, thaumaturgy.
There's also no reason to lock it down to one element. Fae, druids, and Native American shaman will likely use nature magic, an earth wizard would tear open a portal in a cave or tunnel, a water wizard might find opening a portal behind a waterfall easiest...in my opinion, spirit is the least likely. After all, the Nevernever isn't exactly a "spirit world", though it does have some of the trappings.