tomBitonti
Hero
You're thinking like a RW human- how would the caster or Imp know that a particular weapon "blows up?" Could they even recognize the weapon for what it is?
And modern bombs are no longer big piles of gunpowder; many of them can only be detonated by an electric charge, not heat, fire or impact. And there's not going to be ways to detonate them just laying around; they're designed to be dropped or launched, not blown up on base.
I'd say that Telepathy and Charm Monster would let the Erinyes know how to make stuff blow up.
I think this answers a lot of the rest:
Their biggest defense is probably simply the opponent's lack of understanding of modern capabilities.
That is, the big problem here (for both sides) is knowledge. There will be hugely different results depending on whether both sides are unknowing, knowing, or if one side knows but the other doesn't know their opponents capabilities.
If the Erinyes came across years before the invasion, that would be a big deal. Or, if both sides started the fight expecting the same sort of opponents as themselves.
A big problem would be either side's actual ability to even comprehend the others capabilities.
Here's a different scenario: Lawful Evil cleric, 11'th level, goes undercover and offers to provide healing (in particular, Heal) to recruit turncoats. At 13'th level, you can add in Regenerate and start giving folks back lost limbs or organs.
Thx!