This can be done either in Story or in Free Roam. If done in Story you will have to purchase this weapon from the ammunition store when it becomes available for $6000. You will also be able to obtain this in a later story mission which will automatically provide you with a BioClaw. If done in Free Roam you will have access to this weapon from the start.
You will need to knock over one pedestrian into two or more pedestrians. Find a group of three or more pedestrians, for this achievement it works a lot better if the group is very close to each other.