I never assume it's as simple as "choosing a [specific element on mission reward]"

Well, it is. It's not so simple if you're concerned about element and prefix. But if the only requirement is that it's slag (which is all Maya needs for a Rubi), then that's quite easy to obtain through dashboard farming. It shouldn't take more than around 10 attempts.

Speaking of assumptions, the very mention of an "editor" is an assumption in itself, since I never said—or even implied—that a person would feel the need to use one in order to obtain a slag Rubi.

Think what you want about whether or not the slag Rubi is a valuable weapon for Maya. But since OP is asking about good gear for UVHM, it would be foolish to not mention a weapon that's so easy to obtain, a weapon that slags enemies all the same (regardless of its level requirement), and a weapon that has ridiculous synergy with Cloud Kill (to a point where it's pretty much guaranteed to fully restore Maya's health in about one second).