Maybe I am just not seeing it, but I don't think I can find anything stating that spirits are all now immune to natural weapons/have hardened armor/etc. None of them have the power in their stat blocks, for certain. Can someone point me to where it says they do?

Effectively, the
critter has a Hardened Armor rating equal to twice its
Essence against that particular kind of damage (see
Hardened Armor, p. 397). This means that if the modified
Damage Value of the attack does not exceed the
Immunity’s rating, then the attack automatically does
no damage. If the modified DV exceeds the Immunity
rating, perform a Damage Resistance test as normal,
adding the Immunity rating to the dice pool for this test.
Additionally, half (rounded up) of the Immunity rating
counts as automatic hits on this test.
Some Immunities function slightly differently, because
the attack they protect against doesn’t do damage,
per se.

Immunity to Normal Weapons: This applies to all attacks
that are not magical in nature; weapon foci, spells,
and adept or critter powers function normally. If the critter
also has the Allergy weakness, then the Immunity
does not apply against non-magical attacks made using
the allergen.

Interestingly only the Fire & Water Elemental lists an elemental weakness to their opposite element, so using a fire hose on a fire elemental or napalm on a water elemental will work as it's their allergy.

he he - I don't think a fire hose will do much against a fire elemental. now a water hose...

semantics.

And shouldn't you be off somewhere getting ninja'd?

A fire hose refers to the hose used on a firetruck that firefighters use to put out fires with.

But I can see your point, it could be better named.