I second or third or whatever the successive round thing. Hits, fails a con save, gets stunned. If they can manage to hit the same character in the next round (which is going to be difficult with a range weapon while printed, as other have pointed out) then you can kill, if you hit again, and if they fail another con save. But ultimately it's going to boil down to the GM/encounter as to whether the weapon is accessable in the first place. And, of course, we also have to remember the things like a Sarcophagus could exist too. Instakill in that sort of world may not be so bad.