sorry, could not resist the topic title...
so, there's this little problem with spirits being invincible to just about everything other than magic. by RAW, about the only type of summoned beast a mundane can actually do immunity-bypassing damage to with starting gear is a water elemental (with the flare gun). However, there are few problems that can't be solved via the application of a much larger explosive device. i think the following recipe proves to be technically legal:
1-2 kg of high-rated foam explosive (for arguments sake, rating 12, 1kg, say)
wireless blasting cap
A long, reasonably sturdy stick
shape foamed explosives into a shaped charge. apply blasting cap. place on end of stick. perform touch attack. detonate wirelessly via free action. probably a pretty injured spirit. alternatively, apply to suicide drone and perform remotely.
by RAW, the spirit's armor rating is halved by the direct contact, and since it's a shaped charge you're safe from the blast. the minimal damage of this little bundle of joy is 13P, maybe better if you roll well on your demolitions setup. adding in another kilo or uprating your explosives gets you up to 16-20P or better. even your tougher spirits probably don't want to put up with that on a regular basis.
on a related subject, it seems like one defensible reading of the immunity power would be that the immunity rating is actually equivalent to the essence of the creature, not the doubled virtual armor number. therefore if the (doubled) essence of the creature is penetrated by the attack, the creature would get its unmodified essence added to its damage resistance test, plus half its essence in free hits. this seems a lot saner than the intepretation that it's free hardened armor. a force 6 spirit would still get 12 soak dice and 3 free hits against any physical attack, for example, and that attack would still have to beat 12DV + AP to work in the first place. it would make them hard to kill physically, but not so ridiculously hard as they are now. i don't even think this qualifies as a house rule, though definitely rules-lawyering...