Make sure you're not visible and behind your target. Also some thieves cannot backstab due to the kit they are equppied with. Can't remember if he is, but if Yoshimo is a swashbuckler he will not be able to backstab. Other than that you should be able to do it. My thief (level 15 I think) regularily deals out 50 or so damage with a backstab using a +3 longsword. I know for sure that 2 handed weapons don't work but the shorter stuff should (including katanas and scimitars).
Anyway good luck
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