Steel Devil can be powerful if you can leverage it's 'trick', and work around it's weakness. Stealth and SPSitV elevate it to something really vicious. Better than Ebon Shadow or Thrown in my opinion for assassin type play.

You open with a strong withering ambush-attack, preferably against what appears to be the squishiest opponent, and chain in Steel Devil Strike (or Empty Mind Strike). If you add SPSitV's Fatal Stroke Flash you are basically making a huge decisive attacks without resetting. Odds are really good you incapacitate your opponent and can use stealth to remain hidden...