[Major Doom]

I'm playing with a group and I was told that a non-magical character can attack a spirit using weapons with elemental properties (for example, a taser).  Now I looked through the core book (Spirit Combat, SR4a, page 186), read the Special Types Of Damage on Acid, Electricity, and Fire (SR4A, pages 163-164), also read through the Spirits chapter in Street Magic, and on top of that, I've read a few threads here that mention electricity or gel rounds (?) can be used by mundanes to fight spirits.  Unless I missed it somewhere, which could be possible since lot of rules are spread out within a book and among books, I can't seem to find where it says a non-magical character can use elemental weapons against spirits to offset a spirit's innate Immunity To Normal Weapons power.  Can anyone point me in the right direction?

[Mäx]

You can use any weapon what so ever to attack spirits.
Those with high damage and/or AP are preferred against spirits of higher the force 2-3.
Elemental effect doesn't let you ingnore the spirits ItNW but it's AP -half does help a lot as it halves the armor provided by the power.

[Dead Monky]

If none of your guns work (you need bigger guns) you can always use Attacks of Will.  But trying one of those is basically suicide against a powerful spirit.

EDIT
Fixed me a typo.

[Mäx]

If none of your guns work (you need bigger guns) you can always use Attacks of Will.  But trying one of those is basically. suicide against a powerful spirit.

Yeah if first you don't succeed, get a bigger gun and if your gun was already a rail rifle, then you didn't have a chance in the first place and should just content with kicking the ass of your GM.

[The_Gun_Nut]

I have yet to find anything a rail rifle can't fix.  Even a force 12 spirit gets nervous around those.

[Mäx]

I have yet to find anything a rail rifle can't fix.  Even a force 12 spirit gets nervous around those.

Hence my comment to just kick the ass of your GM if rail rifle doesn't work, as the situation was obliviously a premeditated TPK on the part of him.
Rail rifle is after all deadly up to force 13, assuming you can hit that spirit.

[Chaemera]

I have yet to find anything a rail rifle can't fix.  Even a force 12 spirit gets nervous around those.

Hence my comment to just kick the ass of your GM if rail rifle doesn't work, as the situation was obliviously a premeditated TPK on the part of him.
Rail rifle is after all deadly up to force 13, assuming you can hit that spirit.

To be fair, there are GM's who believe in a "persistent world" concept, namely the world exists and continues outside the PC's direct experiences. If the GM decided that a Force 20 fire spirit lives in the abandoned warehouse on fleet street, there's a Force 20 fire spirit in said warehouse. The GM's job at that point is to provide at least some hint that going in said warehouse is not a good idea. Unless they want to make a deal with said fire spirit.

Now, if the GM looked at his plans for tonight's run and said "this fight would be more fun for me with a Force 20 fire spirit", curb-stomp him.

OP, Max has already answered your question; however, if you're looking for how to map it out in the rules:

A critter with Immunity has an enhanced resistance to a certain type of attack or affliction. The critter gains an “Armor rating” equal to twice its Magic against that damage. This Immunity Armor is treated as “hardened” protection (see Hardened Armor above), meaning that if the Damage Value does not exceed the Armor, then the attack auto- matically does no damage.
...
Immunity to Normal Weapons: This immunity applies to all weapons that are not magical (weapon foci, spells, adept or critter powers). If the critter has the Allergy weakness, then the Immunity does not apply against non-magical attacks made using the allergen.

Hardened Armor is even tougher than normal armor. If the modified Damage Value of an attack does not exceed the Armor rating (modified by Armor Penetration), then it bounces harmlessly off the critter; don’t even bother to make a Damage Resistance Test. Otherwise, Hardened Armor provides both Ballistic and Impact armor equal to its rating.

So, a Force 4 spirit has Armor 8/8 against non-magical attacks, and if the attack fails to deal damage greater than the modified armor rating, it's ignored.

Example:
Dude with Light Pistol (4P, 0 AP) shoots at Force 4 Beast Spirit, rolling a net of 2 hits on his attack.
4P + 2 = 6 < 8 Ballistic armor, attack ignored.

Example 2:
Dude with Light Pistol loaded with EX-Explosive (5P, -2 AP) shoots same spirit, rolling a net of 2 hits on his attack.
5P + 2 = 7 > 6 Ballistic armor (8 - 2 for AP), attack affects the spirit & it needs to roll for soak (Body 6 + Armor 6 = DP 12).

Elemental attacks (including electricity) all have Armor Piercing of -half (See pages 163 - 164, SR4A). Electricity is simply the cheapest / easiest one to acquire for your average mundane.

Example 3:
Dude with Taser (6S(e), -half) shoots same spirit, rolling a net of 2 hits on his attack.
6S + 2 = 8 > 4 Impact armor (8/2), attack affects the spirit & it needs to roll for soak (Body 6 + Armor 4 = DP 10)

These examples should make it pretty clear, if you're purchasing on the cheap, a taser or SnS is your friend against spirits. Not only is it the least expensive, it's the highest damage when you hold all other variables the same.

If you aren't purchasing on the cheap, get that gauss rifle, name it Vera and end that spirit.

[Dead Monky]

I'm one of those persistent world GMs.   

[Chaemera]

I'm one of those persistent world GMs.   

As am I. I still remember the time I was running a DnD game, decided where the ancient-as-the-earth green dragon lived. Only to have the party decide within two sessions that they wanted to go explore that cave on the map the found. And they had seen paintings form a previous era depicting said green dragon. And they knew it lived there. And they went anyways. Fortunately for them, they were able to do a job for the dragon in exchange for not being eaten that day.

Before you chastise my players, they'd not played Shadowrun before, they didn't know any better than to deal with a dragon.

[Nomad Zophiel]

Its not cheap, but Arsenal has several venoms from Awakened critters that can go in capsule rounds, supersquirt paintballs or injection darts. Ask your GM, but since its an unmodified venom from an Awakened paracritter power, I'd think it would ignore the immunity entirely. Damage varies from 6-12 and net hits don't add to that but it should put a dent in spirits.

[Major Doom]

OP, Max has already answered your question; however, if you're looking for how to map it out in the rules:

A critter with Immunity has an enhanced resistance to a certain type of attack or affliction. The critter gains an “Armor rating” equal to twice its Magic against that damage. This Immunity Armor is treated as “hardened” protection (see Hardened Armor above), meaning that if the Damage Value does not exceed the Armor, then the attack auto- matically does no damage.
...
Immunity to Normal Weapons: This immunity applies to all weapons that are not magical (weapon foci, spells, adept or critter powers). If the critter has the Allergy weakness, then the Immunity does not apply against non-magical attacks made using the allergen.

Hardened Armor is even tougher than normal armor. If the modified Damage Value of an attack does not exceed the Armor rating (modified by Armor Penetration), then it bounces harmlessly off the critter; don’t even bother to make a Damage Resistance Test. Otherwise, Hardened Armor provides both Ballistic and Impact armor equal to its rating.

So, a Force 4 spirit has Armor 8/8 against non-magical attacks, and if the attack fails to deal damage greater than the modified armor rating, it's ignored.

Example:
Dude with Light Pistol (4P, 0 AP) shoots at Force 4 Beast Spirit, rolling a net of 2 hits on his attack.
4P + 2 = 6 < 8 Ballistic armor, attack ignored.

Example 2:
Dude with Light Pistol loaded with EX-Explosive (5P, -2 AP) shoots same spirit, rolling a net of 2 hits on his attack.
5P + 2 = 7 > 6 Ballistic armor (8 - 2 for AP), attack affects the spirit & it needs to roll for soak (Body 6 + Armor 6 = DP 12).

Elemental attacks (including electricity) all have Armor Piercing of -half (See pages 163 - 164, SR4A). Electricity is simply the cheapest / easiest one to acquire for your average mundane.

Example 3:
Dude with Taser (6S(e), -half) shoots same spirit, rolling a net of 2 hits on his attack.
6S + 2 = 8 > 4 Impact armor (8/2), attack affects the spirit & it needs to roll for soak (Body 6 + Armor 4 = DP 10)

These examples should make it pretty clear, if you're purchasing on the cheap, a taser or SnS is your friend against spirits. Not only is it the least expensive, it's the highest damage when you hold all other variables the same.

If you aren't purchasing on the cheap, get that gauss rifle, name it Vera and end that spirit.

Thanks for the detailed explanation, but this is where I am confused.  Physical Spirits automatically receive the Immunity To Normal Weapons critter power, which is also treated as Hardened Armor.  The Hardened Armor part I get, but when reading the RAW for Immunity To Normal Weapons part under Immunity, especially the following:

This immunity applies to all weapons that are not magical (weapon foci, spells, adept or critter powers).

This for me says, anything that is not magical in nature, be it bullet, explosive, taser, battery acid, menstrual blood, cow's piss, photon torpedo, etc., does no damage.  But if I had a pair of chopsticks that are weapon foci, then just the Hardened Armor part applies.  Yes, no?

[Nomad Zophiel]

This immunity applies to all weapons that are not magical (weapon foci, spells, adept or critter powers).

This for me says, anything that is not magical in nature, be it bullet, explosive, taser, battery acid, menstrual blood, cow's piss, photon torpedo, etc., does no damage.  But if I had a pair of chopsticks that are weapon foci, then just the Hardened Armor part applies.  Yes, no?

Nope. If you had a pair of chopsticks that were weapon foci then ITNW provides NO armor.

[Muspellsheimr]

A critter with Immunity has enhanced resistance to a certain type of attack or affliction. The critter gains an "Armor rating" equal to twice its Magic against that damage. This Immunity Armor is treated as "hardened" protection (see Hardened Armor above), meaning that if the Damage Value does not exceed the Armor, then the attack automatically does no damage.

What about that is not clear?

Immunity to [STUFF] provides Hardened Armor vs. any attacks made with [STUFF]. It has absolutely no effect whatsoever on attacks made with [NOT STUFF].

As Immunity provides the effects of Hardened Armor, it follows all the rules for Hardened Armor - including being modified by Armor Penetration & the half Impact effects of most Elemental damage & Gauss weapons.

[Chaemera]

Yeah, "Immunity" was poor word choice. When they say it has "immunity to non-magical weapons", they're talking about immunity as defined in the preceding sentence (Hardened armor equal to magic x 2).

They aren't talking about "immunity" as commonly understood.

It is worth noting that the Oxford English Dictionary defines immunity as follows:
"protection or exemption from something".

So this would fall under "protection", rather than "exemption". Personally, I'd have used a term like "Resistance to Natural Weapons". Less possibility for connotation to create confusion.

[The_Gun_Nut]

The word "immunity" is a hold-over from previous editions, where the creature was actually immune to mundane weapons.  Force of will attacks was all that mundanes could do against the spirits.