Just hoping to get a concensus on this, or an official answer, but a "Staggered" enemy is considered any with damage equal or greater than its Stagger value. It's very possible, especially with the chainsaw, to get that stagger down to 0. No rule seems to deny this, just wanted to make sure it's totally legit to, say, play Enthusiasm, followed by any other movement skill, to just start ripping off Imp heads without damaging them first. With Rip and Tear, you'd go up to Cacodemons without issue. Combined with Adrenaline Rush popping into your hand and maybe the basic bonus actions giving movement, you'd totally smash the crap out of a whole swath of stuff without even rolling dice.

In theory and given the perfect setup I suppose so. Having not played the game yet, how often that happens and does it really make a difference; I don't know.

I am changing my answer. Per the RRG (PDF pg.6),"If a demon has suffered damage equal to or greater than its stagger value, that demon becomes staggered." I think a demon has to suffer one point of damage to become staggered. This ultimately makes it that stagger value essentially can only be reduced to 1 at the lowest.

Would love to hear some other perspectives because I am still trying to grasp some concepts of this game. Stuff like this is pretty different from the mechanics of other FFG games I have played.

In another forum other people are saying the demon doesn't have to suffer damage. Your earlier interpretation may be the correct play.

Even though I don't have the game myself, I can confirm from one of the videos for the demo, they in fact stated that weapons like the chainsaw can reduce a monster's stagger value to 0, enabling Glory Kills even when no damage has been inflicted (I don't know how many of those abilities can stack however, in the video, the player chainsawed through several Pinkies, Possessed Soldiers and even a Mancubus and Baron of Hell without actually damaging any of them). This ability alone obviously makes the Chainsaw a very valuable weapon, and likely to be everyone's personal favorite where Glory Kills are concerned.