Okay, I finally got it! Heres what I did...I started a new file and flew through the story. When I got to sequence 4 and after the memory where Ezio dresses up as a minstrel, I left the palace grounds and stuck to the roofs and I finally got the tax collector on the southeast side of the Hagia Sofia while I was headed toward the arsenal. Hope that helps whoever is looking to get that trophy out of the way.