Gets harder once folks start getting more HPs, shields, can cast Word of Recall, and such, but pre-35 thieves are capable of killing in lots of different ways.

As long as you can hide and know your way around Thera, you can catch folks off guard and get some cheap kills with any thief build pre-20. Afterward you really need some of the thug or binder skills. There are lots of cheap tactics to take: the dirt, merc rescue, circle and trip approach, jack and scroll 'em up, the ever-lame knife and flee, hide, sneak, knife and flee approach (it works) and of course good ol' backstab, trip/cheap shot. Bind legs will generally keep the person in battle at least longer than your backstab lag then you can trip n' chase.