/run if GetUnitName("target")==nil then TargetNearestEnemy() end
/run if not IsCurrentAction(8) then UseAction(8) end
/Cast Sinister Strike
/run UIErrorsFrame:Clear()
Place auto-attack on bar-spot 8 (or change the macro to whatever fits your purpose)
This targets nearest enemy, starts attacking and casts SS. Swap for backstab if daggerspecced