Gae Bolg only ever killed mortals though. Even if, say, it had the conceptual ability to impart death onto all subjects, the problem with Rui-Ling is that his immortality takes the shape of 'automatic resurrection after death'. It's not killing Rui-Ling that's the problem. It's the 'automatic resurrection' that needs to be taken care of somehow, but currently, yea, Lucille's the only one that can keep him dead, and even then, that's only until she tires of lugging his corpse around with her everywhere.

In regards to geis, [url=https://en.wikipedia.org/wiki/Geis]there's a similar problem...[/url]
[quote]A geis can be compared with a curse or, paradoxically, a gift. If someone under a geis violates the associated taboo, the infractor will suffer dishonor or even death.[/quote]

Kek, with all the money that Chikage's throwing around, I wonder if someone will eventually just straight up purchase the Dangeki Cosplay Shop. XD