Just took me less than 10 tries to get that crossbow with the method described above :D
But I had to use a greatbow. There are "lines" on the "Patches bridge", btw, and if you go to the last 2 lines, you'll have a good sight on the knight.
Just try to aim for his head or a bit above that and he'll "really" fall down. If he drops something, just equip your Silver cat ring and you'll suffer no damage by jumping down. Of course, it is highly recommended to have the Symbol of Avarice and the Crystal Sage Dagger equipped. Coins could also help. But yeah, I got it in under 10 tries, so good luck with this method :)