I think a thief can actually be a fairly effective assassin hunter in the mid ranges. Sub-40. After that, trance and a thief's general falling off power-wise skews it pretty hard in the assassin's favor.

Basically if I were to play an assassin-killer thief I'd go the thug/third attack route. And I'd use haste scroll + mace/dagger. Vulns obviously if you can get them.

I'd try to knock out the assassin and get a look at him. A thugs maledicts are underrated, especially in the low-mid ranks. Then you have a choice. Either haste, and knife/backstab > cheapshot permalag ftw. Or, If he's not covering stats at all, I might be willing to get into a gouge/kidney shot fight with him. You should be able to get your maledicts off a bit better than him. Kote and kans can be a bit unreliable in the mid ranks, just be prepared to dip if he lands his stuff on you.

A hasted thug thief, I think, can definitely blitz down assassins. Hit fast and hard, don't be scared to flee if they kote/kans you, and inhale haste scrolls like they're candy.