Hm, I tried this one before release and it worked, but some unrelated code was later changed that apparently made it no longer trigger. It's crazy how this one is detected--it was too complex to do another way for a simple achievement, so it literally reads the message log to look for the sneak attack confirmation
(but the message has to appear in just the right place...)
In any case, I've updated it for Beta 7 and it should work again, though I'll add it to the list of a couple impossible achievements from yesterday.