Wield daggor in mainhand, continue knifing until blindness dust hits, it will still put out more damage than the assassin can toss out in melee unless he throws without trying to do kansetsuwaza. Kot + kans, succeed or fail = three knifes, which on top of a backstab is around 70-90% of that assassin's HP, slightly less if there's some basic dam redux involved but not overly much.

If thug thieves don't scare the fuck out of a given assassin, even a veteran one, they should. Bad ones do not change this fact. A good thug thief gets a way higher body count than a good assassin for precisely the fighting style that rapes the assassin.