Use the Throw Sand skill to stun four guards at once to unlock this achievement or trophy, worth 10 gamerpoints or a Bronze trophy.
The first step to unlocking this achievement/trophy is to learn the Throw Sand skill in the practice field in front of Villa Auditore in Monteriggioni. Speak to the highlighted trainer near the practice field and select “Unarmed Combat” under the “Special Moves” category. Practice the Throw Sand skill three times to add it to your arsenal.
To perform the Throw Sand skill, hold the X/Square button while unarmed and engaged in combat. Hit a guard with the sand and he will stagger on the spot for a few seconds.
To unlock this achievement/trophy you have to stun four guards at once. So, start by getting a sizeable group of guards after you. It might help to throw a Smoke Bomb down before throwing the sand. Though you can’t use the Throw Sand skill while not engaged in combat, stunning the guards with a Smoke Bomb can give you some time to get some distance from the group so you can hit them with the sand as they approach.