See Venetian Gladiator for more info. Once you finish the mission "Novella's Secret", the icon will appear right on your map in Florence. Go there, and complete the mission for this achievement. This mission is very open as there are no levers or timed parts in this mission, you just have to climb and jump your way to the end. The entire area is open from the get go, so it can be easy to get lost if you aren't paying attention. If you get lost during all of the jumps, just try and look for the pigeons, they will show you where you have to go.