Kill 10 guards with a single arrow storm.

Go to DNA and restore Memory Sequence 1: A Roll of the Dice. Find Salai, Leonardo’s assistant at the Theives Guild and bring him back to Leo’s workshop.

Go inside the building on your left, once inside skip the cinematic and you will find yourself in front of another building. Head inside and you will find some people gambling on the left. Go to them, skip the cinematic.

Then there will be 10 Robed men attacking you, walkthrough them and use the Arrow storm to kill them all.

You will earn a bronze trophy for the PS3 or 20 Gamerscore for the XBOX 360.

You can watch the video here