I'm thinking if heaven does do go the guerilla route, then so what? The "collateral" damage will just go to hell. Asides from the propaganda hit, all they'd accomplish would be angering the humans even further.
I also wonder how the discovery of an afterlife will change the public's perception and threshold for casualties.
We'll have to wait and see, but I expect that a number of things will spin off. One would be "death briefings" at hospitals, particularly veterans-only hospitals. What to expect when - not if - you die, how to ascertain your location when you arrive in Hell, where friendly, human-controlled areas are in relation to your current position and how to make your way back to them... Survive, Escape, Evade, Resist as re-written for Hell.
Expect recruiting offices to be going up in all human-controlled areas soon.
Hmm... I wonder.
Years ago, there was a book about efficient design. Lost the title, unfortunately. One of the things it went into detail about were small, cheap, expendable one-channel radios that were powered by the heat of a single candle, durable enough to be air-dropped, cheap enough to build by the hundreds of millions, designed to be built inside of a common tin-can.
Radio Free Hell, anyone?
And what about the old FP-45 Liberator pistol?
If that makes a comeback, the demons are going to seriously regret ever angering us. A cheap, disposable pistol that, by itself, can't drop a demon. But it can injure one, maybe kill it if you get a head shot. And it's better than no weapon at all.
Stamp out a few million, air drop them wherever there's a going human resistance...
Come to think of it, most of the "nasty surprise" inventions that came about during the darkest days of WW2 would apply. Even the Nazis, bastards that they were, had good ideas about cheap & cheerful, easy-to-produce weapons intended to make occupation duty a misery.
The EMP-44 comes to mind, though that might be a bit much, demons with brains could turn captured ones against humans.
Knives, though... lots and lots of knives. Cheap but durable machetes, short swords, smatchets...
Lieutenant-Colonel William Fairbairn, the man who literally stamped his name on the modern fighting knife, is probably down there in Hell as well. I suspect he'd find it vastly amusing to be supplied with copies of the knife he made famous.
PS: Behold the Power of Google. Found it. "Design for the Real World" written by Victor Papanek, a simple radio built around a tin can and a candle. He passed away in 1999 (I wasn't aware of that. Pity, he was a good man.)