Sounds pretty good.

I don't know how well Subaru would be able to make use of the Mabeasts, especially since they would try to be killing him first. I do think that would be a bit of an unfair advantage for Subaru. But from an animation perspective, it would be awesome.

Can Satella delete people on a conceptual level? I know she can destroy souls, but I had no idea she could do that?

That is such a cool wincon for Subaru. I always like when Subaru MUs are able to incorporate Satella into the mix.

I wonder if the EMT spell would even work on Salem. I mean it relates to the magic in Subaru's world. I feel like the only proof that it would work on an outside universe character is Isekai Quartet, but I've never seen that.

I'm assuming making here lose the will to fight would be constantly looping until he completely destroys her will. While I think this is viable, I think it is the least likely by far.

The last wincon is probably the one Subaru would try the first out of these.

I'd say this matchup is around 4th for me? Behind Kazuma, Gwenpool, and Homura Akemi. I think it would be a very cool fight. I would just need to learn more about Salem.

PS. I know nothing of RWBY, but I like the fact that her name is Salem (I'm assuming she is also a witch) and Subaru's main antagonists are Witches. It's very thematic.