This is a story-related trophy and cannot be missed via normal campaign progression.
After taking a quick tour of Venice with Leonardo, you will need to go make a new friend. A woman who bumps into you is ultimately shot in the leg with an arrow, so you must escort her to Antonio.
Eventually you will meet Antonio and he will ask for you to do some small tasks (gotta earn his trust, right?). Do these and you will be preparing yourself for the final mission in this sequence.
You must eliminate 5 archers at the beginning of the final mission and then you can proceed to the target. Scale up the building he is in and position yourself to show him your blade. The trophy will unlock after you meet up with your new friends and the cutscene that follows.