Would it work? The spell itself specifically targets a demon and hits its Might. I'm inclined to say that if you want to destroy the Might of a non-demonic entity you'd need to design a spell for such creatures:
Perdition of the Infernal Supplicant PeVi General
R: Voice D: Momentary T: Individual
Similar to Demon's Eternal Oblivion, this spell targets a being with Infernal Might that is not a Demon, and destroys its Might equal to the spell's level if it Penetrates.
Base effect, +2 Voice
I'm inclined to do this with the other beings as well, making two or more versions of the Might Stripper spells. One that effects the 'pure' inhabitants and one that effects beings with Might. The distinction might have to be that the 'pure' examples exist as Spirits naturally, but possibly can embody themselves with a power, while the others are naturally embodied creatures that also have Might. For example, you use Lay to Rest the Haunting Spirit on a ghost, but would use an entirely different spell on a dragon or giant with Magic Might, and still maybe a another spell for a natural spirit with Magic Might. Not a perfect system, but it does hamper the Might Stripper spells a bit and reflects more on the Art of magic rather than the science and adds new layers to the bestiary.
Doesn't work with Faeries, since there is no such distinction. Faerie Might makes you a Faerie, corporeal or not. I wouldn't distinguish in regards to the spell when it comes to the other realms either.
Our house rule to "fix" might strippers: Remove the two bonus magnitudes from the guideline.
Faeries might be the exception to this I guess. The one exception would be beings that gain a Faerie Might score but were not native to it. but such a distinction is so specific and/or rare it's not likely worth it. I was going to say there are ghosts with Faerie Might, but they might just be faeries acting like ghosts. So because faeries are so unique compared to the other powers, you mgith just need the one type of Might Stripper for them and it works on all examples of them.
If you don't look at RoP: Infernal, but just core you'll see that there is no distinction between demons and creatures with infernal might. They are one in the same according to core. RoP: Infernal added other types. I would "House Rule" that DEO work on all creatures with infernal might, just like it did in core.
Might stripper spells, like Demon's Eternal Oblivion, IMO, are realm-aligned. A creature with infernal might is, for all intents and purposes a demon. A creature with faerie might, is a faerie, and a creature with magic might is magical. I prefer to think of the spells that strip might as realm aligned, rather than being aligned, otherwise one would need hundreds of spells to be effective against all the possible creatures one might encounter.
See that's why I don't have a problem with it. "Why didn't my spell work?"
"You used Demon's Eternal Oblivion, that was an infernally corrupted wolf. You needed a different spell to affect it."
It would basically be a group of maybe 6-7 spells total.
Infernal: DEO and a spell for embodied creatures with Infernal Might but not actual demons
Faerie: One spell, like Skewer the Petulant Faerie. I just don't think there's anything with Faerie Might that isn't a faerie.
Magic: A spell for embodied creatures, a spell for ghosts/spirits. Though I could see one spell for ghosts and another for spirits to be really specific; a spirit is not a ghost.
Divine: ...Not a good idea. But if you really want to...One to kill angels and one to kill Divine creatures.
It's not really necessary, I can grant that. I just kind of think that it goes with the Art of magic. You want to kill demons, make a spell that kills demons, but don't expect it to work on a giant corrupted lizard.
The thing is with just core that giant corrupted lizard? Demon. It wasn't until RoP:Infernal came out If you start going by creature type in RoP:Infernal... well a phantasm isn't a corrupted beast or a demon. I guess that's a third spell. And really, what about ghosts? Those aren't demons either. What about a demon blooded? Do those count as corrupted beasts? Blarg. I suppose you could set a dividing line of corporeal and non-corporeal. That's 8 spells. Unless you get to one of those artificial "spirit bound into an item" magic might types that a few forms of magic can make. Then its all confused again.
Core clearly intended the guideline to be by realm.