Yeah, I know, a lot of forma. But a prototype of this build with only the umbral mods, prime [Reach] and [Killing Blow] can already deal with the steel path, also the umbra forma helps not only get all the mods in place but also allow you to do a similar build but for normal attacks if you feel like it. You'd normally use [Primed Pressure Point] or, more likely, [Condition Overload] instead of the sacrificial one and the rest is some status, crit dmg, etc.
The idea for this specific build is use the shadow's 1 to gather enemies and then heavy attack to instakill them. Apparently this weapon has 100% slash chance on heavy attack, meaning if anything manages to somehow survive, will die from the slashes.

You can actually change [Buzz Kill] by anything you want, including [Jagged Edge], [Carnis Mandible], etc, and if you don't want to spend that extra forma you could even put [Jagged Edge] R4 instead. You'll loose a bit of damage but it will most likely not be much of a difference.