usually when he sees demons he breaks out this... massively damaging spell, magnus exorcismus, which only effects demons and undead things. it's like super holy, and though it takes a while to get all its damage out, can pretty much kill anything it catches in one round (in terms of usual monsters, obviously, boss monsters are tougher).

that's how it works in canon, at least! i'm super cool with tweaking it however we want to, if he ends up using it on her. BUT THAT SAID, depending on how the thread goes, he'd pretty much bust that spell out as soon as he realizes whoever he's talking to is a demon. unless they've just been rescuing orphans or something idk.