Like all these other guys said, get the training so you can do it. after, go to Venice - San Marco. Once you get there, immediately in front of you are 4 guards, 1 with a spear. Wait for him to check out the hay bales and shanky shanky, take his spear and run across the bridge straight to the right. if you grab the pair that are further down the road, turn straight around and head back to the bridge, but to the left is a corner, go in there and its like a little pocket. Because its so close together its much easier to get this achievement here than anywhere else ive tried. Bring medicine, trust me.